Editor – someone who makes an edit, irrespective of how large the contribution is
Author – contributor of original work (remark: in this context, "originality" refers to "coming from someone as the originator/author")
Copyeditor – editor who improves style and accuracy (but not content) of an article (not being member of the Wikipedia Cleanup project)
Quality and Reliability
Recent changes patrol – vandal fighting; initial quality control
Reviewer (Sighted versions, only Wikipedia editions w/ FlaggedRevs turned on, e.g. German Wikipedia) – people involved in a light-weight validation process, flagging articles that are free from vandalism
Editorial staff member (German Wikipedia only) – subject matter expert; the editorial team has the right to delete inadequate articles, regardless of the normal deletion process, see w:de:Wikipedia:Redaktion Biologie for an example
Member of the Zedler medal jury (German Wikipedia only) – see w:de:Wikipedia:Zedler-Medaille (only one community member, others: members of the Academy of sciences and editors of a scientific journal)
Featured article director or delegate (English Wikipedia only)
Flickr Image reviewer (Wikimedia Commons only) – Trusted user who reviews images imported from Flickr for license compatibility.
PD Image reviewer (Wikimedia Commons only) – Similar to above but verifies validity of public domain license tag on uploaded images.
Bot account operator – Individual, usually with some programming experience and perhaps an account on the toolserver who operates an account which does automated processes like cleanup, conversion, and other maintenance activities.
Rollbacker (not in all language versions, e.g. not on the German Wikipedia) – someone entrusted to do automatic reverts of copyedit (generally vandalism) to previous version.
Administrator (Wikiversity: en: Custodian, de: Pedell, fr: Bibliothécaire) – experienced and trusted user who can protect, delete and restore pages as well as block users from editing
Bureaucrat – promote other users to administrator or bureaucrat status; grant and revoke an account's bot status; and rename user accounts
Checkuser – permission to examine user IP information and other server log data under certain circumstances, for the purposes of protecting Wikipedia against actual and potential disruption and abuse (e.g. Sockpuppets)
Oversight – prevent revisions from being viewed by anyone except other users with the "oversight" permission
Steward – users with complete access to the wiki interface on all Wikimedia wikis, including the ability to change any and all user rights and groups
Wikipedia mediation committee member – community members volunteering to assist in resolving disputes between users
Wikipedia Arbitration committee (en: ArbCom, de: Schiedsgericht, fr: Comité d'arbitrage) member – elected editors responsible for conducting the Wikipedia arbitration process, having the authority to impose binding solutions to disputes between editors (does only exist on bigger language editions)
Arbitration Committee Clerk (English Wikipedia only) – editors selected by the Arbitration Committee to assist the committee with procedural aspects of its work