What is the most significant difference between agile development and the historic way software has been written?

This is a rather fundamental question related somewhat to an earlier question asking you to define the agile development process. Interviewers can ask any question at any time during the interview. Even though you may have already addressed this topic, you should maintain your focus and answer the interviewer's questions.

"The primary difference between agile development and traditional software development is that traditional software development used a three-stage process. The stages are design, development, and testing. Agile development uses these same three stages, but each stage is performed within each sprint or iteration of the development process. This enables agile developers to identify issues early before they become major."