I think sessions is now more accurate: with default settings, Google sends in a note to the database once you visit the page, and thus "open" your session. Then, your actions are recorded and assigned to a temporary, unique user number. This is saved on your local memory as well (that's why cookies are needed). The session expires after 30 minutes, so new clicks made after that period are assigned with a new ID.
Normally, it wouldn't make any difference. However, if your page hosts e.g. documentary films (which your users watch whilst being on site), one user can generate a few sessions due to expiry time. Your report would then show inaccurate data.
The same applies when someone opens up the page (even as one of many tabs in browser - we all do that!) and leaves for lunch. Once they're back and resume where they were, they will be counted as a fresh user.
Well, it's just one of the many technical nuances of the Google Analytics system!