Users who do not log in are said to be IP users, or, anonymous users, also referred to as anons. These users may edit pages which are not protected or semi-protected; their edits will appear in page histories alongside the IP address from which they were made. They can also create new pages.
Anonymous editors cannot move pages, upload media, or maintain a watchlist. Because IP addresses are so often dynamic, an IP-based user page is generally impractical, and in fact disallowed on several wikis. Anonymous users are often prohibited from participation in processes such as deletion and permissions debates, or may only participate on a restricted basis (e.g. may leave comments, but may not vote).
They also are not able to select the option of minor edit as this could allow a bad-faith user to make undesirable changes without being spotted for some time.
The public visibility of an anonymous user's IP address means that, in extreme legal cases, the computer from which the user contributes could be identified through contact with the ISP, which can eventually lead to identification of the user. In this sense, "anonymous" users are in fact less anonymous than registered users who do not reveal any personal information.