In a traditional interview, you will be asked a series of questions which typically have straight forward answers like "What are your strengths and weaknesses?" or "What major challenges and problems did you face? How did you handle them?" or "Describe a typical work week."
In a behavioral interview, an employer has decided what skills are needed in the person they hire and will ask questions to find out if the candidate has those skills.
Instead of asking how you would behave, they will ask how you did behave. The interviewer will want to know how you handled a situation, instead of what you might do in the future.