From Seo Wiki - Search Engine Optimization and Programming Languages
A wiki farm is a server or an array of servers that offer users tools to simplify the creation and development of individual, independent wikis.
Prior to wiki farms, someone who wanted to operate a wiki had to install the software and manage the server(s) themselves. With a wiki farm, the farm's administration installs the core wiki code once on its own servers, centrally maintains the servers, and establishes unique space on the servers for the content of each individual wiki with the shared core code executing the functions of each wiki.
Both non-commercial and commercial wiki farms are available for users and online communities. While most of the wiki farms allow anyone to open their own wiki, some impose restrictions. Many wiki farm companies generate revenue through the insertion of advertisements, but often allow payment of a monthly fee as an alternative to accepting ads.
Many of the currently most notable wiki farms got their start in the mid-2000s, including Wikia (2004), PBWorks (2005), Wetpaint (2005), Wikispaces (2005) and Wikidot (2006).