A visit is all the activity that a user does in one sitting. One unique visitor can have multiple visits, and one visit can consist of multiple page views.
A visit ends when a user stops interactive with your site for 30 minutes, has 12 hours of consistent activity, generates 2500 hits, or has 100 hits in 100 seconds: Visit
Consider a user who is going through a checkout process, gets most of the way through and remembers they forgot to add an item to their cart. Then they get to the billing page and closes their computer to look for their wallet. 5 minutes later they log back on but have to start the checkout process again - this time completing checkout. If this was the only user, and you were going to look at page views for each page in your checkout process, you'd see lots of page views in the early stages of checkout, some in the middle, and one at the confirmation. However if you look at visits, you'd see one visit where someone goes all the way through checkout.