Steam is a free program that you can download.
You can download games off their store (usually pretty cheap)... Add friends, check up with what games they have and their achievements.... Instant message them via Steam and also free voice chat.
It just keeps you and your friends connected. No negatives if you ask me.
It is basically like Xbox Live. I don't think it enhances performance or anything like that. It is a digital store and it has some great sales. I don't believe you can turn it off either
You can change your profile status to invisible so you can't be disturbed by friends and stuff.... Steam will run in the background and shouldn't make anything run slower.
Ah, ok. Didn't know you could do that. Although don't notifications still pop up?
It is great for deals and for managing the games you own (in digital format, of course).