What is it?

Well this is what it is. It's a black vinyl disc that looks a bit like a record but without a groove. Well, that's not strictly true as there is a very fine groove over which the playing stylus rides. The information is contained on the surface of the disc as very slight changes in the electrical charge. The stylus rides on top of the grooves in contact with the disc, the stylus being carried on an arm that is moved electronically to follow the information on the disc. A conductive electrode sits close to the surface of the disc on the end of the stylus and detects the minute changes in the charge on the surface of the disc. The signal is amplified and the information extracted to re-construct a TV image.

left click to enlarge This is what you normally see, just the outside of the plastic caddy. There are 2 colours, blue, indicating Stereo-phonic sound and White, indication Mono-phonic sound. Each disc has 2 sides with the UK/PAL version holding up to 75 minutes of video + stereo sound on each side.
left click to enlarge This is the disc that sits inside the caddy. Not easy to see except in real-life is the segmentation visible on the surface. A segment represents a TV frame. The disc rotates at high speed (375 rpm) synchronous to our 50 Hz AC mains frequency.
  The caddy with the disc inside is presented to the slot at the front of the player and pushed fully in. The player unlatches the caddy. The caddy is withdrawn leaving the disc in the player.
  After side 1 has finished, the caddy is re-inserted, withdrawn with the disc now inside, inverted and re-inserted to leave the disc inside once again so that side 2 can be played.
  After side 2 has finished, insert the caddy once again, withdraw and store. No finger-prints, no dust, no dirt, which is just as well because the information is so densely packed that the smallest dust particle can cause the stylus to miss-track. This is visible when viewing and appears as a skipping of frames on the TV.
  This is the GEC/McMichael player. One of the 2 types sold in the UK.
  This is the Hitachi player. The other of the 2 types sold in the UK. They are really the same machine, just the outsides are different.
  This is what they look like inside. Mechanical bits, electrical bits and lots of space.