How does Fluther program the links between a question to a list of appropriate, related topics?

I mean, how when I write here or in any other Questions and Answers web site, how they know the topic of my question and how the topics are managed in DB, because every one can add any topic he wants.

How does it work? I need to program a simple task like that, that creates the combination of some text to some category/topic. What I don’t understand is how I manage the list of topics, How that list is being updated, because there is no way I can think about all the topics available, the list is huge.