The sit-up is an abdominal exercise. Here you lie on your back, the legs are angled. In contrast to the crunch, the entire upper body is detached from the floor. It is easier to fix your feet, either with weight cuffs, barbell bar, etc., so that the feet do not  remove from the floor during the exercise.

Sit-Up: https://www.youtube.com/watch?v=1fbU_MkV7NE

Tipps to do sit-ups: https://www.youtube.com/watch?v=Q-mK2nVl0Ek