Keycap Materials and Legends.
ABS - Acrylonitrile Butadiene Styrene - ABS is one of the most common Keycap materials. Most pre-built retail keyboards use this material for keycaps, and brands like GMK use it, but they use a higher quality ABS. ABS is know for starting to become polished, or shining, after being used for a while. This is because the oils from your fingers are breaking down the plastic slowly (don't worry, it wont break it down enough to ruin your caps)
PBT - polybutylene terephthalate - In general, PBT is seen as superior over ABS keycaps, because it's stiffer, durable, stays true in color, and doesn't develop a shine effect over time. PBT is usually more expensive than ABS, due to the quality.
POM - Polyoxymethylene - POM is like a mix of ABS and PBT. It a similar texture to ABS, but is as durable PBT. It's more dense than both PBT and ABS. POM keycaps are around the same price as PBT keycaps, but much less common.
Double shot - Double shot legends refer to a method of keycap manufacturing that uses a separate piece of plastic for the legends that is the fused with the main keycap. This means you could sand down the top of the keycap, and the legend would still be there. Double shot keycaps are usually more expensive than other keycaps, because of the advantages.
Dye sublimation - Dye sublimation uses a kind of printer and dye to dye plastic a specific color at the molecular level. Dye sublimated legends are very resilient, and wont wear off with normal use. Dye sublimation does have some limitations, though. The legend has to be darker than the main keycap.
Pad printed - Pad printed legends are ink applied with a pad similar to a stamp. The whole keycap is then covered in a layer of clear-coat for protection. The legends on these keycaps are really not durable, but this method is used because its very cheap.
Laser etch - laser etched legends are exactly what they sound like- the legends are etched into the plastic keycap using lasers. Like pad printing, the legends are not durable, but the method is cheap.
A key unit is the distance of one key to the next. Understanding key units becomes important when using custom layouts to ensure proper key set coverage.