So, you want to listen to your music in some sort of digital format, maybe on your PC, on a digital music player, or on a cell phone or PDA. (If you're planning to listen to audiobooks, compress non-musical speech, or compress sound/music for a game or other project, that's a bit beyond the scope of this w/u.) You might be confused by the alphabet soup, unable to tell the difference between the various next-generation audio formats, or wanting to move up from plain old mp3.
Before getting into formats, let's talk about bitrates. Bitrates measure the quality of the music; a high bitrate means less information has been discarded, meaning your music will sound better. (If this doesn't make sense, read about how audio compression works first. Don't worry, I'll wait.) Bitrates are measured in kilobits-per-second, and generally range between 64k and 256k. Two identical pieces of music of the same bitrate (save for mp3pro with its oddball extensions) will have the same filesize. The advantage of a later-generation, more-efficient compression format is that you'll get better sound out of the same bitrate, which means you can either use a lower bitrate (creating a smaller file) to get the same quality of music, or get better sound out of the same filesize.
If you're enough of an audiophile to notice the difference between CD-encoded sound and lossy-compressed sound, take a look at FLAC, a lossless compression format based on the Ogg file format. FLAC files play in many standalone digital players and almost all mainstream PC or PDA music players, and they are completely indistinguishable from uncompressed CD sound (just like a png is indistinguishable from a tiff of the same bit depth and resolution.)
Sometimes you'll also see VBR files. These are files that use a bitrate that varies (hence the acronym of Variable Bit Rate), depending on the complexity of that part of the song. Simple parts will use low bitrate compression, and complex parts will use high bitrate compression. Generally, VBR files with a listed bitrate use that listed bitrate as the maximum bitrate, so a VBR file will tend to be smaller than a fixed-rate file of the same listed rate, with the same sound quality. Most next-generation sound formats will always use variable bit rates.
In general, 44khz, 16-bit quality (which is different from bitrate), stereo sound is assumed. All formats support this, and you'll never want to fiddle with this unless you have some reason to make intentionally low-quality sound files.
Now, bitrates aside, let's get to the meat of things...the formats!
Lowest Common Denominator - MPEG1 layer 3, aka mp3
This is the most common format, supported by every major and many minor platforms, as well as by almost every digital music player device. It is the baseline by which all other formats are compared. In general, you'll never go wrong encoding things into mp3; only the Sony standalone players (Including the Sony Digital Walkman, but not including the PlayStation Portable or the Clié PDAs) don't support it, and many devices, new and old, support nothing else. On PCs, WinAmp, iTunes, and Windows Media Player are popular choices for players, and players exist for Palm PDAs, Windows Mobile devices, Symbian phones, and many other platforms.
On the other hand, mp3 is the lowest-quality choice among the common formats, and almost none of the legal music download services use it due to the lack of copy-protection. (emusic is one of the rare exceptions. So is Russia-based allofmp3, but it is questionably legal at best.) If you want to buy music online, you're going to be stuck with another format. Older mp3 encoders often clip the treble and/or bass, leading to a slightly tinny sound, and mp3 doesn't support more than 2 channels of sound. Also, many older mp3 players don't support certain very high or very low bitrates or variable bitrates.
Most people will be happy with 128k VBR encoding, or 192K fixed-rate encoding. Any higher bitrate will rapidly run into diminishing returns.
A Kludgey Extension - mp3PRO
Fraunhofer, one of the owners of the patents on the mp3 codec, extended their mp3 codec to allow for an extra high-frequency channel to make up for the clipping common to mp3 encoders. This means people with broad hearing ranges or tastes for orchestral music will see some benefit, but it is otherwise identical to mp3. This means that mp3PRO files will play on all mp3 players, although only specifically enhanced players will see any benefit from the extensions.
Unfortunately, this kludge has poor software and device support; WinAmp (with a plugin), Audion, and MusicMatch are the only common players to support it, and only RCA supports it with its Lyra mp3 players (and support is missing in some of the newer, hard-disk-based players.)
The extensions don't add much to the filesize, so if you use Audion, MusicMatch, or RCA's Lyra mp3 players, there's not much reason to not use mp3PRO, as long as you have a good-quality mp3PRO encoder.
mp3PRO files should be encoded at the same bitrates at which you encode normal mp3 files, and they use the .mp3 extension.
Ogg Vorbis is one of the highest-quality formats, and is the only major format without any patents associated with it. The former is valuable to everyone, and the latter is helpful to anyone who wants to sell digital music (as long as they're not worried about copyright protection, which isn't included in Ogg Vorbis). It doesn't really have any major problems, save for the fact that Apple, Microsoft, and most device manufacturers have snubbed it, so iTunes, Windows Media Player, and most digital music devices don't support it. (iTunes can play Ogg Vorbis files with a Quicktime extension kludge, but this is far from optimal.) As long as you haven't tied yourself to one of these formats, you'll probably be quite pleased with Ogg Vorbis.
Ogg Vorbis has a spotty history, because many of the files floating around were encoded with a beta version of the encoder. While Ogg Vorbis is one of the best-quality formats, there are many low-quality files floating around out there, making it a risky choice for downloading files from peer-to-peer services.
Almost all Ogg Vorbis files are encoded 128k VBR, and this will satisfy most users. Ogg Vorbis files will have the .ogg extension, which Vorbis shares with the Ogg Speex speech-compression codec and any future Ogg Project formats.
The Mainstream Next-Generation Choice - Advanced Audio Coding (AAC)
AAC is the sound component of MPEG4, and is Apple's and Sony's choice of next-generation audio compression. It's one of the best-sounding formats (comparable to Ogg Vorbis), and supports copy protection, making it popular with downloadable music services. (Apple uses Fairplay copy protection with their iTunes Music Store, and Sony uses ATRAC3 with their Sony Connect service.
Be aware that "ATRAC3" files are actually AAC files encoded with Sony's ATRAC3 copy protection. Anything that says it will play ATRAC3 files will play non-protected AAC files, and this includes Sony Cliés, Sony's digital music players, and the PlayStation Portable.
AAC support is a bit thin in music player programs, with only iTunes and WinAmp counted as major supporters, but it's the best-sounding lossy format usable on the popular iPod, and can be played on all Sony devices (even the ones without mp3 support.) It also has some oddball support; Nokia has thrown their support behind AAC, for example. It's the best-sounding format for legal downloadable music, as well.
Be warned that many AAC files will have copy-protection, and that files downloaded from Apple iTunes Music Store and Sony Connect will only play on the programs and devices supported by the company in question (iTunes and iPods for iTMS songs, and MusicMatch and Sony players, for Sony Connect files.) Because of the incompatible copy protective, a Sony player won't play a song downloaded from iTMS, despite the fact that they use the same format, iPods won't play Connect songs, and a non-Sony, non-Apple player won't play songs from either.
AAC includes VBR by default, and 64K or 128K encoding is more than enough for almost all listeners. .aac files include just an audio stream, while .mp4 files can be MPEG4 movie files or AAC audio streams in an MPEG4 wrapper.
Well, it's Microsoft's chosen format. It's a next-generation format, quality-wise; not quite as good as AAC or Ogg Vorbis, but better than any of the others. What it brings to the table is unparalleled device support; WMA support for digital music players is second only to mp3 support. Microsoft unsurprisingly owns Windows Media completely, so Windows Media Player and jack-of-all-trades WinAmp are the only common players with support.
Windows Media is popular with downloadable music services; MSN Music, Napster 2.0, Wal-Mart's online music store, and all of the minor stores use copy-protected WMA files. Even better, this copy-protection is supported by almost all WMA-playing devices and all versions of Windows Media Player (including the version included in Windows Mobile, on PDAs and smartphones.) Microsoft is rolling out their PlaysForSure initiative, making it easy to identify devices that will play WMA files, with or without copy protection.
While the device support is surprisingly good, software support is terrible. If you want to play WMA files on a Mac or Linux PC, a Palm PDA, or a Symbian smartphone, you're pretty much screwed. There are WMA players of varying quality on those platforms, but none of them support copy-protected files or encoding WMA files. Windows Media Player on Windows itself is the main way to rp WMA files from a CD. Be careful to disable copy-protection when doing this, if you plan to share your files on a peer-to-peer service or transfer your WMA files to a device that doesn't support copy-protected files.
WMA is usually encoded using "good" encoding, which is 128K VBR. Occasionally music services use 64K or 192K encoding, but this isn't common. WMA files have, of course, the .wma extension.
The Also-Ran - Real Audio (RA or RM)
Included mostly for completeness's sake, Real Audio is mostly a leftover from when it wasn't forgone that mp3 was going to become the most common audio format. Real hasn't gone away, though, and the format in its current version is about the same quality as WMA. It was also one of the first formats to support copy protection, for what it's worth, and is still used for streaming (instead of downloadable) music.
Real Audio has wretched software support. Right now, only Real Player (which, in its defense, has improved since the bad old days of spyware and ads and bloat) and ever-extensible WinAmp support it. Device support is terrible, as few standalone players support it. (Real has written a hack for the iPod, called Harmony, that allows RA files, with or without copy protection, to play on the iPod, but Apple may release a firmware update that disables this at any time.) Real Audio does have something of a cult following among Palm and Symbian users, as the Real Player for those devices is surprisingly good, and free to boot.
While Real Audio supports copy protection, only Real's Harmony music service uses it, and Real Player Pro (the only common end-user Real Audio encoder) doesn't force copy protection on you.
Real Audio is usually encoded in "Standard," which is encoded at 128K. (I'm not sure if it's VBR or not.) Real Audio files are either .ra or .rm, although it shares the latter with Real Media movies.
Updates and corrections would be appreciated. Thanks to unperson, dogganos, and randombit for corrections and updates. Last updated: November 9, 2004