A "founder" is someone who created a wikia. Founders do not have any special authorities on a wikia, and they do not own the wikia that they founded. What they do have, however, is administrator and bureaucrat rights, as well as the ability to set the early direction for a community.
Founders are automatically given administrator and bureaucrat access to their wikia, but the purpose of this is to allow them to help get the wikia started through customization, not to give them control over the content. Any other regular editor of the wikia can also become an administrator or bureaucrat. This can happen either by asking the founder and/or the community of the wikia or by adopting the wikia if the founder has since become inactive.
Determining a wikia's founder
One way to determine who the founder is (and whether they are active) is to visit Special:ListUsers and search for "Administrators" and "Bureaucrats." Oftentimes, admins with both user rights are the founder. On larger wikias, you may see a few users with both rights. This does not mean they are the founder, as only one user can found a wikia.
The best way to find out who the actual founder of a wikia is to check each admins page. The founder of the wikia will have the founder badge on their profile.