I think thats a brilliant deal just on the numbers i dont know anthing about that amp though

right then,
solid state/transistor/digital/"normal amp" - these all use digital electronics to create the sound and all those names meen the same thing, these are usually the cheapest too.
Valve/tube amp - more expensive, use valves/tubes suprisesuprise, these are the best amps, you get more natural overdrive, distortion, gain, clean...... , they basicallly sound much nicer. Also they are alot louder than a solid state amp of equal wattage.
Valve state - these are hybrid amps, part solid state and part valve so they are cheaper to make but sound more like a fully valve amp, the valves are usually just in the preamp stage.

Stacks
erm im not too sure abut this but i think a full stack is a amp head (valve or solid state) driving two cabinats
and a half stack is a amp head (valve of solid state) driving one cabinet

hope that helps