It depends on who asks. If it's someone who genuinely cares about me and is articulate about story, I chatter on until it looks like they might like a change of subject. With Tom (my husband who is discovering new reserves of patience as time goes on), I talk until he forcibly changes the subject. Hey, he bends my ear about baiting hooks for his long-line fishing, so I figure cosmic justice is served.
Other than that, I have a variety of responses: "I write fantasy--you know, dwarves and wizards and such." Or if they're more thoughtful sorts: "I really write about unlikely heroes and survival." Then I shut up unless they indicate more interest, and even then, I don't like to push it. But if they ask more questions, I'll give them another short answer. I don't want them to be trapped, like someone who didn't see the Jehovah's witnesses coming in enough time to hide behind the couch.