Well, the main functions are to read from and write to the ECU. This is a Cobb Accesport (Version 1), and I don't know all of the technical specs. Cobb tends to be very closed-lipped about such things. This, as far as I remember, is only compatible with '04-'05 Imprezas, but there is firmware/software available to recode it for other models.
Either way, this isn't something that you'll be able to just plug in. It needs to be reflashed/reset before it can even be used for its original purpose. For people who have a car that this would work with, their best option would be to send it in and (hopefully) get a discount from Cobb on their current, Version 2 Accessport.
You can read more about the Accessport on their site:
CobbTuning.com - AccessPORT