WorldCat is a service offered by OCLC. It started in 1971; at the time, it was designed to serve 54 college libraries in Ohio. Today, it is a gargantuan database of bibliographic information (the world's largest, mind you!) that contains nearly 48 million records representing works in over 400 languages. Something new gets added every 15 seconds.
A WorldCat record generally contains a physical description of an item and information about its informational or artistic content. Some records also contain cover art images, tables of contents, etc.
While most of the holdings are books and music recordings, some of the more unique items in the database include:
There's all manner of coolness in the thing. Catalogers have been known to get all squishy just thinking about it.
If you are an author, getting your book listed on WorldCat is important. Why? WorldCat is the other main source of information about books that websites like Bookmooch and LibraryThing refer to. A WorldCat listing goes a long way toward making sure potential readers know your book exists. Furthermore, if your book's not in WorldCat, to the librarians of the world it's as if your book just doesn't exist. And since librarians can be some of an author's strongest allies, you want to make sure they can easily reference your work.
How do you get a listing in WorldCat? In theory it's pretty simple: just make sure that at least one Worlcat-member library immediately gets a copy of your book when it comes out. But two or three member libraries are better. For more information, read How books get into libraries.
For more information about WorldCat, visit its website at http://www.oclc.org/worldcat/