Per NCAA rules, the school is allowed to give athletes a maximum of three tickets to each sporting event that is issued through a pass list. These have to be regular season events within a thirty mile radius to the campus. Basically what that means is the school cannot be giving tickets to recruits of their bowl game or NCAA tournament game. Unofficial visits can occur at any time.
The difference between that and an official visit is that the college programs pays for you as an athlete to take that official visit. They can fly you in or reimburse you for your driving expense included in the visit. There are also more rules surrounding the official visit that includes food, lodging, and other specific monetary amounts that are enforced by the NCAA.