If you're looking on a tight budget, head down to your LBS and see if they have some used wheels in the back. Chances are that they'll have something that's good enough for campus riding.
You'll need 26" wheels and yes, you'll need a cassette on the back wheel. You want to make sure the cassette has the same number of speeds (probably 8 or 9.) You'll also likely want to get a set of wheels that have approximately the same rim width, so you don't have to severely adjust your brakes when switching wheels. (not an issue if you're running disc brakes.)
As for tires, get a slick or a semi-slick 26" tire. Your LBS will likely have one of these, too.
Did I miss anything?
You'll need 26" wheels and yes, you'll need a cassette on the back wheel. You want to make sure the cassette has the same number of speeds (probably 8 or 9.) You'll also likely want to get a set of wheels that have approximately the same rim width, so you don't have to severely adjust your brakes when switching wheels. (not an issue if you're running disc brakes.)
As for tires, get a slick or a semi-slick 26" tire. Your LBS will likely have one of these, too.
Did I miss anything?