A common question to ask in Google Analytics is:
How many times have my pages been viewed?
Sessions are counted only once: A user comes to your site and google analytics will trigger a visit. So, for sessions with only one page viewed, sessions and pageviews will be the same. Now, if a user navigates on your site to another page, the number of sessions will still be one, put you will get two pageviews, one for page A and one for page B,
If you want to know how many times a pages has been viewed, you need to use Unique Pageviews instead of Sessions.