Bureaucrats are senior editors. As well as being full Administrators, they have management abilities where they can grant limited user rights.
Administrators are trusted editors, equipped with the tools to keep the wiki running smoothly. They can issue blocks to troublesome editors, revert spam/vandalism using the rollback tool, and delete spam pages. They are authorized to use the tools at their disposal to deal with issues facing the wiki to the best of their ability.
Bots are special accounts that are used to make automatic edits in a short period of time.