Probably the easiest difficult question you will face, and it is usually one of the first questions. Typically the interviewer is looking for an overview of your professional experience. Provide an answer that shows a logical progression in your career moves. If your educational background has led you in a certain direction, bring that into the overview. It is important to stay positive about your past experiences.
Nobody wants to hire a person that complains about past employers.
They figure you may complain about them in the future. Talk about why it was time to move on, that you learned a lot, etc. Employers want to know that they are considering someone that will go above and beyond the call of duty. It is easy to hire a person that will do what they are asked to do between nine and five. However, it is better to hire a person that does that and more.