Interviewers asking what motivates you during an interview are attempting to gauge your personal and professional character. They are attempting to drill past obvious questions such as your greatest academic or professional accomplishment. By asking what motivates you they're asking you to reveal things about yourself. This line of questioning is very important. Your answers should sound genuine and unrehearsed. Consider questions about motivation as a great opportunity to sell yourself to the employer.
Tell a short personal story about yourself as you describe what motivates you. For example, talk about how you learned about work ethic at an early age from your parents, and how that has remained with you through the years. Don't get too sappy with with personal stories, but also don't be afraid to open up and talk about yourself.
Tell the truth about what drives you and give examples. If you're motivated by money, say so -- if it's the right kind of job. People applying for positions with a charitable organization usually aren't looking to get rich, so talking about being motivated by money probably would not fit in that setting. On the other hand, interviewers from an investment firm on Wall Street or from a major law firm might love to hear you talk that way. Keep your responses truthful but tailor them to the audience.
Talk about a fear of failure if that's your motivation and tell how that has helped you in your academic or professional life. For example, describe how as a student you left high school unprepared for college statistics classes, but succeeded by seeking help from tutors and putting in lots of extra work. Use the example to show your willingness to press ahead despite odds because you do not like failing.
Talk about the satisfaction you feel when you get things right, and how that drives you to give your best all the time. Explain how you enjoy positive comments from a boss or a co-worker on a job well done, and how recognition from people you respect also encourages you to work harder. Use this opportunity to talk about special awards or company competitions you have won. Don't brag, but do discuss how winning awards addresses your competitive desires.
Discuss how you try not to make mistakes, but you realize everybody does, and how you try to learn from your experiences. Describe a situation in which you made an honest mistake -- and how you showed leadership by admitting to the mistake and offering a solution that solved the problem. An example could include mistakenly going far over budget on ordering products or supplies, but solving the problem by sharing some of the expense with another department. The key is to illustrate how you quickly recover from a bad decision.