Plank, or front support in Pilates, is a well-known exercise. It is one of the most popular exercises for developing core strength and stability.
While Plank really targets the abdominals and shoulder stability, you will find that plank is an excellent way to get a full body challenge. In order to do Plank properly there must be integration of all the core stabilization muscles. The arms, glutes, and legs are active as well.
Plank can look like the up part of a regular push up. But, in most cases, a regular push up entails much more strain in the upper body -- especially in the shoulders and neck -- than plank in Pilates or yoga.
You may want to begin with a modified version of Plank and work up to the full version, especially if you are weak in the upper body or have neck strain issues. Please see the plank prep exercise.
You will know you're doing plank well when you have good form, feel your center working, and have good shoulder stabilization yet are not incredibly rigid.