In the simplest terms, git pull does a git fetch followed by a git merge.
You can do a git fetch at any time to update your local copy of a remote branch. This operation never changes any of your own branches and is safe to do without changing your working copy. I have even heard of people running git fetch periodically in a cron job in the background (although I wouldn't recommend doing this).
A git pull is what you would do to bring your repository up to date with a remote repository.
