Laserdisc is a video disk format from days long gone. Just like with Beta and 8-Track it has gone to format graveyard. It was a mildly successful consumer format since it was the only high quality read-only video format for most of the last quarter of the 20th Century. Surpassed by DVD but borrows many of the same features and improves on them.
Laserdisc technology has been around since the 1970s. In fact the modern CD(the ISO 9660 format) and CD Player are direct descendants of this format created by Philips and such the technology is very similar. A laser pickup is used to read pits (which are smaller than the wavelength of visible light and causes the nice rainbow effect when inspected) and reproduce a very precise and strong reproduction of the video and audio signal.
Laserdiscs came in various sizes the most popular was the 12 inch disk. Both sides on a Laserdisc could be and often were used to store information. Early players could not read both sides without user assistance(ie. go to the player, eject the disk, turn it over, press play) but later models had "automatic flipping mechanisms" which were often just another laser pickup.
The "modern" Laserdisc is made of a sandwich of acrylic or polycarbonate and glue and something to protect the data layer. This "protective layer" seemed to be a highly guarded competitive secret since this was the thing that protected the disk from "laser rot". Like creeping crud "laser rot" consumes and destroys the disk by oxidizing the material. Once oxidation sets in the laser no longer reflects and the signal is lost. There are known Laserdiscs made of glass and aluminum but these were never consider "consumer grade".
The two encoding formats were known as CAV and CLV. CAV in simple terms are encoded like CD-ROM with circular tracks while CLV is encoded like audio CDs and LP where the information is read from the inside out(like a record needle following the groove of a record).
CAV gave many improvements over CLV. The signal-to-noise ratio is better because as a feature of all disk formats the linear space where you can store information increases from the inside of the disk to the outside. It was not unheard of to get 2db better signal on the outter edge of the disk side. CAV also gave simpler time base correction and offered true "frame encoding". The big draw back to CAV stores less video and audio signal. CAV disk side could only hold 30 minutes of video while a CLV disk could hold 60. Another drawback is that your Laserdisc player had to spin the disk at a constant 1800 RPMs while CLV does not. This unfortunately generates a lot of background noise because 12" disks aren't light.
One important limitation of Laserdiscs is that it is not a true digital video format. In simple terms video is still a pulse of FM signal encoded in the pit-to-pit spacing and the audio is encoded in the difference between the pit-nonpit parts. Not to mention the resolution was absolutely fixed as was the signal encoding. A PAL Laserdiscs can only reproduce a PAL video signal.
Regardless Laserdiscs offered a clear advantage for videophiles at the time. For NTSC you can often get LDs with 400x482(luminance lines x visible scan lines) where with VHS you would often only see 250x482. Like other disk media there is no wear by just playing and handling properly(unlike tape formats). There is no Macrovision or region locking(ignoring PAL vs NTSC) on LD that you find on DVDs today.