Official visits are paid for by the university. This includes transportation, room, meals, and entertainment (kind of ambiguous there but being in Atlanta helps us here) for you and your family (up to 3 people total.) I believe only seniors can take official visits and they are only allowed 5 total visits. To be eligible you must first send the college your transcripts and SAT/ACT test scores
Unofficial Visits can be taken at any time at the expense of the player. If it is during a dead period the player cannot talk to any coaches. They are allowed up to 3 free passes to a home game during their visit and number of visits isn't regulated. This is how we get many Atlanta area elite prospects in our stadium who aren't considering GT - free tickets.