This question seems simple, so many people fail to prepare for it, but it is crucial. Here is the deal. Do not give your complete employment (or personal) history. Instead give a pitch one that is concise and compelling and that shows exactly why you are the right fit for the job. Start off with the 2-3 specific accomplishments or experiences that you most want the interviewer to know about, then wrap up talking about how that prior experience has positioned you for this specific role.