A visit is a series of requests from the same uniquely identified individual (a visitor) in a set period of time. A visit is made up of several hits and pageviews and often shows a track through a site.
Visits are usually defined as a set period of time, typically 24 hours or 1 hour. For example, if you go to a certain site 3 times in the same day, if their visit period is 24 hours, your 3 pageviews will be ascribed to 1 visit. But if their visit period is 1 hour, and you visited in 3 different hours, your pageviews will be logged as 3 different visits. It's important to know what the log analysis software defines as a visit period.
Visits are calculated by setting a cookie or web bug on the customer. Some Web analytics packages will calculate visits based on Web logs, without setting a cookie, but these, while useful, are not going to be as accurate as the cookied method. Because of dynamic IPing and other anonymizing techniques, non-cookied visits are based on educated guesses.