The sig makes me think this is a spam post, but I'm bored and going to explain anyway
The stages are all a marketing ploy for companies. Everybody has a different interpretation. I usually just go by the Cobb stages because that's what is widely used.
Basically, stage 1 is a general retune of the stock car without any other mods. Typically a tuner will smooth out the tune for better drivability.
Stage 2 consists of a downpipe and a tune. A downpipe really frees up power in a turbocharged Subaru and the tune is necessary. Some companies add in an intake at this stage but not all.
Stage 3 really isn't all that well defined, it usually means a swapped turbo of some sort. Usually a small turbo upgrade but there isn't really a clear definition for this stage.
I know when I had my WRX the big thing was "Stage 2.5" which was an STi turbo on the WRX.
Anyway, hope that helps and welcome to the boards.