About.com is a database of how-to guides and informational pages. Unlike crowd-sourced guide services, About only uses guides written by credible experts it's hired. About picks its guides to write in certain subject areas. Their work is then overseen by a professional editor who does copy editing, fact checking, and offers advice on improvements.
About's business model revolves around advertising and being picked up in search results. Each topic page has its own contextual ads which are usually related to the topic. About can also sell advertisers space on certain topic pages, so they'll always appear.
The only user-generated aspect of About is its forums, where users can discuss articles, or any other topic with other About.com users and guides.