Pixel art is, as the name suggests, Computer Generated Imagery created from pixels. There is a common misconception that, since all computer graphics are made of pixels, then any art created on a computer is then pixel art. In particular, people want to attach the title of pixel art to any kind of 3D Graphics since they are rendered on a computer. This, though true to a certain point, doesn't fit in with the definition of pixel art as art created pixel by pixel using minimal colors, graphical tools, special effects, and anti-aliasing.
Following that definition, pixel artists most generally use a very simple image editor so that the more interesting tools and effects of larger and more complex programs can't distract or tempt them while they work (although I know of many that prefer using Photoshop simply because they don't like the feel of MSPaint). Some pixel artists start with a rough template, especially if they're trying to create an array or sprites for a game or just to entertain themselves, but most others start their art from scratch. Despite being described as being drawn pixel by pixel, it's not uncommon to use a pen tool to draw long lines for a rough outline, or the fill tool to fill an outline with a base color. These are simple tools that help make something rather time consuming a lot easier, so that the artist can focus their energy on the more important element of pixel art; the pixels.
Once they have the basic outline and the base colors laid out, pixel artists, like most other artists, will focus on one area at a time. If they're creating a sprite, they might focus first on shading the skin, starting with a color slightly darker than the base color to fill in areas of shade, and then moving down and adding darker details until they reach their desired darkness (or lightness, if working on highlighting) and texture. If a pixel artists is working in an image editor that supports layers, it's not uncommon or against any unspoken rules to allow layers to help speed their piece along. Otherwise, using a simplistic program without layers, the artist is forced to have a steady hand while shading to gingerly place each pixel in its proper position without going out of the line.
As for how one goes about shading and highlighting, that's completely up to personal taste and style. Many, including myself, do simple cell shading which is creating blocks or blobs of color in the position that should be in shadow, while many others swear only to use dithering in whatever they make. Still others think of pixel art as very tiny pieces of digital pointillism. Either way is perfectly acceptable and all bring extra spice into the world of pixel art.
When working with a community of pixel artists, it's not surprising to find at least one person that wants the beautiful and whimsical style of tiny pixel art, but doesn't want to spend their afternoon working on a single piece. These people, though they may not realize what they're doing is the most important unspoken rule of pixel art, will often use airbrushes, soft edged brush tools, filters, or even such evils as soften, smudge, and blur. If they're lucky, they might escape the community and the rabid artists with a small portion of their dignity intact, because from my experience, a good sized portion of all pixel artists are savagely proud of their form of art (though not every pixel artists will chase you down with a torch and pitchfork. Many are quite happy to tell you what you're doing wrong, but in every group, there are the handful of people that aren't afraid to torture you until you leave). This might seem ridiculous to anyone that doesn't create or intimately know pixel art, but if you sit down and spend hours, days, weeks, and sometimes even months to finish a piece of art a fraction of the size of other art forms, slowly clicking and pushing each pixel into just the right position, choosing each color you use individually, and spending half of your time standing four feet away from your monitor to see if the colors and shapes meld together, you would come to understand the blood, sweat, and tears that every pixel artist gives for their art.
To many, these little pieces of art seem rather simple and tend not to give them a second glance. Little do they know how much work truly goes into these tiny creations. Each pixel to a pixel artist is akin to the brush stroke of a painter, or an herb to a master chef. If one is misplaced, the entire piece will look off, and so most of the time and energy a pixel artist spends on their art is in choosing exactly the right position for each and every pixel. When you come across a piece of pixel art that has been put together just right, fitting together like the pieces of a puzzle, it tickles your brain and makes your eyes want to dance. Even to a person unfamiliar with the beauty of pixels, they are wonderful pieces of eye candy.
Most people are more familiar with pixel art than they may realize. During the Golden Age of video games, the only form of art available to the game artists at the time was pixel art (and, though I can't for the life of me find it, I'm fairly sure it had an official name then, other than pixel art), which they used to create beautiful background tiles and lively sprites. Even today, the games you find playing on any hand-held gaming console(save a few cleverly designed games) use pixel art to capture trees, skies, people, weapons, and just about anything you can think of. Pixels are more flexible than you might imagine.
Pixel art is rather rare as far as art goes, so relatively speaking, it doesn't come in labeled categories. But if you're observant, you'll quickly find out that there are different styles and forms that can be grouped together to make your life easier. The most common of those styles are:
- Isometric pixel art has been growing in popularity because of its crisp, clean lines and simplistic detailing. The most common objects done in this style are houses and buildings, and anything of a general square shape. Some artists create isometric sprites, or little people, but they tend to look very much like Lego men. I imagine it would be hard to walk around if your whole body was square.
- Free form pixel art is basically a gathering of art made without grids or templates. Beautiful pixel paintings, strange and unique sprites, and just about anything else that is created from the ground up.
- Sprites are tiny characters or objects usually associated with video games, but many pixel artists love creating sprites for websites, web comics, or just for their own entertainment. Sprites can also be free form pixel art if they haven’t been created from a template, but many artists use one if they’re trying to make all their sprites look alike in body structure.
- Cyberpets and dollz are very much like sprites, only they tend to be much larger and slightly less complicated or detailed than their pixelated cousins. Cyberpets are usually one form of animal or another drawn and colored whimsically and set on a webpage for others to "adopt" for their own page. Dollz are very much the same, only they're almost always in human form and their parts are sometimes interchangeable. Don't like the blue hair? Swap it for the blond wig and put a pink bow on top. Both of these are almost never seen as true forms of art, however, and don't get much appreciation.
- Background images and tiles are hard to find by themselves, but as soon as you start up that old game of Mario or Final Fantasy, you're surrounded by bright green grass, grey brick walls, and shimmering blue skies. Most of these backgrounds are tiles, little pieces of art repeated over and over so that you never have to go without lovely scenery as you run around and battle evil. However, in more stationary games, the background is a single, large image that will always stay the same, giving the artist a chance to make it exceptionally beautiful and detailed. Once in a while, you'll see a beautiful background image by itself as a piece of art, but usually these pieces are meant for games or web comics.
Of course, there are probably a few dozen different styles and forms of pixel art that I missed, but it is so uncommon that it's nearly impossible to find more information than I've pulled together here. Pixel art is slowly coming back to life after dying off when the SNES was replaced with the Gamecube and PlayStation, but there are still so many people that don't have a clue as to what it is. Maybe someday they're realize how magnificent it really is.
If you're still curious about pixel art, check out some of these links.
http://www.deviantart.com - Deviantart has a great pixel art section if you're willing to weed through the miscategorized pieces.
http://www.eboy.com - Eboy is one of the best isometric pixel art galleries out there, and a great starting point for anyone interested.
http://pixelfreak.com - Pixelfreak is another great website, and has an indepth tutorial.
http://finalredemption.com - Final Redemption has a great sprite gallery and is worth taking a look at.
http://www.cosmicjive.org/sweetwater - Sweetwater Valley is one of the oldest and most popular cyberpet websites on the net. They're retired now, but you can still see and download their art.
http://www.leezys-pixelations.com/index2.html - Leezys Pixelations is a lovely pixel dollz website.
http://oh.ekaki.tripod.com/ - O! Ekaki! is my favorite MSPaint and Oekaki tutorial website. Not exactly pixel art, but very close.