A display that makes use of plasma.
When voltage passes trough the gas (usually a neon/xenon mixture), it forms weakly ionized plasma, which emits UV radiation. This UV makes the phosphors emit visible light.
These phosphors work in a similar way as those in a regular CRT TV or monitor, but operate at a lower voltage. Therefore, in order to produce enough light (to yield a bright image), they draw a lot of current. This causes the well-known problem of burn-in, which together with the high power usage is the main disadvantage of plasma displays (not forgetting their high price of course).
Three of these (colored) gas / plasma cells make up one pixel in a PDP. Plasma cells are difficult to make in small dimensions, so that is the reason you don't see small PDPs.