The challenge of maintaining stability and control for an airplane, which moves freely in three dimensions, is closely related to similar forces facing swimmers in the water. Because a swimmer is suspended in and not on the water, a change in any one of the three types of motion affects the other two. The three types of rotation needed to minimize resistance and enable control for airplanes and swimmers are Roll, Pitch and Yaw. Imagine three lines running through an airplane and intersecting at right angles at the airplane's center of gravity. Rotation around the front-to-back axis is called roll. Rotation around the side-to-side axis is called pitch. Rotation around the vertical axis is called yaw.
Swimmers need to be mindful of all three areas when correcting their body positions during the four strokes. To minimize resistance and maximize propulsive forces, try to:
1) swim as flat as possible from head to toes, even during the extension in breaststroke (minimize pitch)
2) avoid over-rotating the hips in freestyle and backstroke (minimize roll)
3) resist crossing hands and arms past the body's midline (minimize yaw)
Also see Streamline Focus section of this USMS article.