what is the difference between a pull request by a collaborator and contributor in GitHub? I don't find any special privileges for collaborators.
The one special privilege a collaborator has over a contributor is... that they (the collaborator) can push directly to your repository (since you have added them as "collaborator").
They can push their own branch and do a pull request from withi your repo, between their branch and master.
As opposed to a contributor, who has to fork your repository first, and make a pull request from their own fork/branch to your repo/master.