What is the difference between author and committer in Git?

That's not really two authors - that's an author and a committer. The two fields have different meanings. The author is the one who created the content, and the committer is the one who committed it. When you do a normal commit, you are both. (And both come with an associated email and timestamp.)

But they can become different in a few key ways:

• git format-patch / git am - this pair lets you turn commits into patches, generally submitted by email, then have someone else apply them. You remain the author; the person who applies them is the committer. This is pretty definitely what happened on github there.

• git commit --amend, git rebase, git filter-branch - These are all basically variants on history rewriting, ranging from single commit to some history of a branch to the entire history. They can potentially modify the committer information - in particular, they always rewrite the committer timestamp. The original author remains in place (in default modes of operation), and if the author is also the one doing the rewriting, their name and email stay, but the timestamp is naturally different.

There aren't multiple authors associated with that commit (nor is it currently possible to assign multiple authors to a single commit). In this case, gliese1337 was the author, and felixge was the committer. Most likely, this occurred because gliese1337 submitted a pull request which was accepted and then committed by felixhe (the repository owner). That workflow's pretty common on GitHub. This is also helpful for instances when a project maintainer receives a patch via email, so the author of the patch itself still receives credit for the patch, even if he or she doesn't have commit access to the project.

A couple of related links:

Short Git Wiki section on author attribution
A feature request for multiple author functionality in Git core