This is not an invitation to recount your life story and all your hopes and dreams.
In fact, it's just the opposite. People use ask this question not because they desperately want to learn about your life, but because they want to see how adept you are summarizing a lot of information in a short amount of time. Do your best to keep your answer to a minute, two at the most. This is your elevator pitch you should have memorized to sell yourself, and most of it should be about professional accomplishments instead of personal details. Throw in a few of the latter for good measure, but in the end they care about what you will bring to the company, so focus on results and recent experience.