The differences between git pull and git pull origin are:
git pull= git fetch origin + git merge origin/master (or whatever your upstream is)
git pull origin = git pull (as long as the origin is your upstream remote)
You can configure an upstream for a local branch. After a new clone, you will have a local branch "master", a remote "origin" and your master branch has "origin/master" as upstream. Your upstream configuration can be seen with git branch -vv or by looking at .git/config.