* Oakland is where Pitt is at. It's got tons of college kids running around everywhere, with tons of bars and places to eat.
* Shadyside is where all of those kids go after they graduate. It's where I'm sitting right now. More families, but still a bunch of stuff to do. Can be expensive (oh wait, you're coming from NYC/SF, it's dirt cheap, nevermind) if you don't look around.
* CMU is located on the border between Shadyside, Oakland, and Squirrel Hill.
* Squirrel Hill is where most CMU students live. It's a very family oriented neighborhood, lots of quiet streets, but still has a quite bustling 'main street.'
* The South Side is a bit far from those three neighborhoods, but it's got a _crazy_ nightlife. The number of bars would be illegal if they weren't grandfathered in, no joke. This is where AlphaLab is, along with a bunch of other tech companies, and lots of families, punks, etc. Reminds me of the Mission a bit, though I was only there for a day or two, so that could be a slightly poor characterization.
What you have to remember about Pittsburgh is that each neighborhood is a totally viable mini-city. They all have a 'main street' where most of the commerce is, and then housing around it. So there's at least something to do everywhere, though some places are obviously more bustling than others.
I've lived here all my life, so if you have any more specific questions, ask away.