What is the difference between `git fetch origin` and `git remote update origin`?

In response to a question about pulling one commit at a time from a git repository, I was recommended to use git remote update instead of git fetch. I have read both man pages but cannot say I understood either in its entirety.

Can anyone explain to me how git fetch origin and get remote update origin behave differently?

It makes no difference when used like this.

remote update is a very high-level command - it supports grouped remotes (remotes.<group> = <list>), and updating all remotes (except those with remote.<name>.skipDefaultUpdate set), but not any of the more specific options of fetch. Under the hood, though, it does the exact same thing as fetch with the default options.

The answer recommending remote update instead of git fetch was actually recommending it without a remote name, just for the sake of fetching all, not just the one named as an argument. This is equivalent to git fetch --all.

I should add the caveat that fetch and remote update didn't actually use the same codepath until v1.6.6.1 (released December 23 2009). Even before that, though, they did essentially the same thing, just using different code (possibly behaving slightly differently in corner cases, but I can't think of any off the top of my head).