Simply, git pull does a git fetch followed by a git merge.
You can do a git fetch at any time to update your remote-tracking branches under refs/remotes/<remote>/.
This operation never changes any of your own local branches under refs/heads, 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).
git pull brings a local branch up-to-date with its remote version, and update your other remote-tracking branches.
Git documentation: git pull
