This is your opportunity to bring your resume to life for the hiring manager. It is usually easiest to start from the beginning of your career, working to your most recent experience. Be sure to include your reasons for leaving previous jobs, if applicable. Give a solid overview but avoid lingering on this question for more than a couple of minutes. You don't want to ramble!
"I began my career with Company X right after graduating from University. In that role, my biggest accomplishment was being promoted after just two months time. I left there after five years when Company Z offered me a bigger opportunity. I've been there ever since. My proudest accomplishments in this role include increasing our employee retention rate by 24% and decreasing accidents by 12% overall."
"I am a recent graduate from XYZ University and completed my practicum with Company ABC just last month. I mentored under the QC Manager there for three months and learned a great deal on CAD/CAM. I look forward to gaining more career wins with your organization."
"I have been a Quality Control Inspector for the past nine years. I started with Company ABC where I worked my way into a team lead position. In my current position, where I have been for ten years, I manage a team of eight junior technicians. I look forward to gaining further leadership experience in this role."