Now, I'll tell you the answer. It's often called a stump or elevator speech.
And do it in 30 seconds! I'm not joking. As a past media expert I wrote many television and radio commercials. The rule of electronic media writing is three words per second. You have 90 words and thirty seconds to answer the question.
Interviewing is often stressful, but the key to controlling your stress is feeling in control. Be prepared. Research the company and the person(s) interviewing you. Arrive early. Have extra copies of your resume, something to write on and two pens. Your stump speech helps to set the tone for the interview, comfortable, complete, direct, succinct. Ask questions by asking for more information, i.e. that's very interesting, could you tell me more. Or, focus on a single issue and ask for clarification or more information, i.e. that's interesting, how does that work exactly, what processes are followed...
Talk less than you listen. My rule is to talk 20 percent and listen 80 percent. The more the interviewer talks the better he/she will likely perceive the quality of the interview.
Finally, if you leave feeling it was enjoyable, it was fun, it was probably a good interview. Read all my interviewing tips by clicking on the article link above. In it we discuss telephone interviews, restaurant interviews, multiple person interviews, what to wear at the interview and more.