History comparison reports highlight the changes between two revisions of a page.
Wikis are generally designed with the philosophy of making it easy to correct mistakes, rather than making it difficult to make them. Thus, while wikis are very open, they provide a means to verify the validity of recent additions to the body of pages. The most prominent, on almost every wiki, is the "Recent Changes" page—a specific list numbering recent edits, or a list of all the edits made within a given time frame. Some wikis can filter the list to remove minor edits and edits made by automatic importing scripts ("bots").
From the change log, other functions are accessible in most wikis: the Revision Historyshowing previous page versions; and the diff feature, highlighting the changes between two revisions. Using the Revision History, an editor can view and restore a previous version of the article. The diff feature can be used to decide whether or not this is necessary. A regular wiki user can view the diff of an edit listed on the "Recent Changes" page and, if it is an unacceptable edit, consult the history, restoring a previous revision; this process is more or less streamlined, depending on the wiki software used.
In case unacceptable edits are missed on the "Recent Changes" page, some wiki engines provide additional content control. It can be monitored to ensure that a page, or a set of pages, keeps its quality. A person willing to maintain pages will be warned of modifications to the pages, allowing him or her to verify the validity of new editions quickly.
Critics of publicly-editable wiki systems argue that these systems could be easily tampered with; while proponents argue that the community of users can catch malicious content and correct it. Lars Aronsson, a data systems specialist, summarizes the controversy as follows:
Most people, when they first learn about the wiki concept, assume that a website that can be edited by anybody would soon be rendered useless by destructive input. It sounds like offering free spray cans next to a grey concrete wall. The only likely outcome would be ugly graffiti and simple tagging, and many artistic efforts would not be long lived. Still, it seems to work very well.
The amount of vandalism a wiki receives depends on how open the wiki is. For instance, some wikis allow unregistered users, identified by their IP addresses, to edit content, whilst others limit this function to just registered users. What most wikis do is allow IP editing, but privilege registered users with some extra functions to lend them a hand in editing; on most wikis, becoming a registered user is very simple and can be done in seconds, but detains the user from using the new editing functions until either some time passes, as in the English Wikipedia, where registered users must wait for three days after creating an account in order to gain access to the new tool, or until several constructive edits have been made in order to prove the user's trustworthiness and usefulness on the system, as in the Portuguese Wikipedia, where users require at least 15 constructive edits before authorization to use the added tools. Basically, "closed up" wikis are more secure and reliable but grow slowly, whilst more open wikis grow at a steady rate but result in being an easy target for vandalism. A clear example of this would be that of Wikipedia and Citizendium. The first is extremely open, allowing anyone with a computer and internet access to edit it, making it grow rapidly, whilst the latter requires the users' real name and a biography of themselves, affecting the growth of the wiki but creating an almost "vandalism-free" ambiance.
Many wiki communities are private, particularly within enterprises. They are often used as internal documentation for in-house systems and applications. The "open to everyone", all-encompassing nature of Wikipedia is a significant factor in its growth, while many other wikis are highly specialized.
There also exist WikiNodes which are pages on wikis that describe related wikis. They are usually organized as neighbors and delegates. A neighbor wiki is simply a wiki that may discuss similar content or may otherwise be of interest. A delegate wiki is a wiki that agrees to have certain content delegated to that wiki.
One way of finding a wiki on a specific subject is to follow the wiki-node network from wiki to wiki; another is to take a Wiki "bus tour," for example: Wikipedia's Tour Bus Stop. Domain names containing "wiki" are growing in popularity to support specific niches.
For those interested in creating their own wiki, there are many publicly available "wiki farms", some of which can also make private, password-protected wikis. PeanutButterWiki, Socialtext, Wetpaint, and Wikia are popular examples of such services. For more information, see List of wiki farms. Note that free wiki farms generally contain advertising on every page. For those interested in how to build a successful wiki community, and encourage wiki use, Wikipatterns is a guide to the stages of wiki adoption and a collection of community-building and content-building strategies.
The English-language Wikipedia has the largest user base among all wikis and ranks in the top twenty among all websites in terms of traffic. Other large wikis include the WikiWikiWeb, Memory Alpha, Wikitravel, World66 and Susning.nu, a Swedish-language knowledge base. The largest wikis are listed and updated on Wikimedia's "meta" wiki.