Visit = A single visit, sometimes called a session, is when a visitor accesses the website and visits any number of pages within a specific time period, before leaving the site. This is the up and coming best metric for analyzing website traffic.
Pageview = A single page access. A single visit can consist of 10 pageviews if the visitor surfed ten different pages on the site before leaving. Pageviews used to be the most favored method for analyzing website traffic, but they are losing momentum in favor of unique sessions.
Hit = When a file is accessed from the web server. A hit is any type of file - html page, image, etc. If a particular page contains three images, then when that page is viewed, four hits are registered - three for the images and a fourth for the html page, itself. Because the number of hits is directly related to the number of media objects on a page, they are not accurately used to analyze website traffic. However, they are used to monitor server bandwidth and stress levels (ie how many hits a server must process at once on a busy website).