The plank (also called a front hold, hover, or abdominal bridge) is an isometric core strength exercise that involves maintaining a difficult position for extended periods of time. The most common plank is the front plank which is held in a push-up position with the body's weight borne on forearms, elbows, and toes. - source
On the other hand, there are various plank exercises which train the different abdominal muscles more efficiently. For example side planks are great to strengthen and tone obliques. We have listed all the plank exercises here.