Extensions are features and add-ons developed for MediaWiki, the software platform that Wikia runs on. They are designed by third-party developers (that is to say, not Wikia or MediaWiki staff) for the purpose of either expanding the function of a wiki or overriding a built-in MediaWiki feature for customization purposes.
See Category:Extensions for help pages on Wikia Help. Note this article may not be completely up to date.
Wikia does offer a family of extensions called "Semantic" extensions. The Semantic MediaWiki suite allows for annotated data from one wiki page to be pulled and queried on other wiki pages. For more about what Semantic is, please visit the SMW website's "Introduction" page. In order for any extension in the package to work, the parent Semantic MediaWiki extension must also be enabled.
Due to software upgrades, Wikia is not able to offer Semantic installation at this time. (Wikis that already have it may keep using it but unsupported. It is working passably well on Familypedia, for example.) You may still request SMW for your wiki at Special:Contact, where you will be added to a waiting list and re-contacted when the extension is again available for installation.
Grants users with the appropriate permission the ability to check user's IP addresses and other information (This is generally a staff-only tool. A few wikis have a few users with this access, but it is not requestable.)