Big
shiny round things that have
films stored on them.
They are 12 inches in diameter, store 1 hour of analog video per side in CLV mode, or 30 minutes (36 mins for PAL) in CAV mode. They a stereo PCM digital audio track (or occasionally DTS), and in the case of NTSC discs, also two mono analog audio tracks. (Used for commentary, or for skaffy people who don't have a digital LD player, or for a Dolby Digital track.)
Better than VHS and VCD but not as good as DVD.