I think certificate programs are typically done as part of an AA or an AS degree, sort of like the equivalent of a concentration for a bachelor's degree. That's how it is done at my local college at least. I believe they can also be obtained on their own, as a way of improving your skills on a specific topic or program. If they are done on their own though, they don't provide as much general/background knowledge as they would when combined with an AA/AS.