In the default git workflow you pull the changes from the origin repository by
merging. This happens automatically when you do
git pull. When you made a
commit to your local branch and someone else has pushed some new commits to the
remote branch this creates a merge commit. This is needed because git needs to
combine the changes from both branches.
I've seen this cause a few problems. The first is that this creates a lot of merge commits, which are basically a lot of noise when you're looking at the history. The multiple (unnamed) branches can also be confusing.