the big thing you will want to look at when buying a compressor is the CFM rating...this number tells you how much volume of air the unit can produce...

compressors that produce a higher volume of air will be able to run more demanding tools...or even multiple tools at once...so you will want to start by looking at which type of tools you plan to run...then you will need to buy a compressor that will be able to produce enough air to run them...

if you are just looking to use air nailers...then the pancake or twin stack compressors are probably what you will want to look at...and the main difference between these two types of compressors is that the twin stacks generally produce 1.5-2 times as much air

just post again if you need any more help finding the right compressor for your workshop