Planes of human motion
Dave Draper of IronOnline has a fantastic and easy to understand summary of the planes of human motion, including the sagittal, frontal (coronal), and transverse planes:
Sagittal plane motion would include forward and backward motions, like sit-ups, back extensions or biceps curls. The sagittal plane cuts through the center of the body, so the motion is front to back or back to front, including straight forward running. Squats involve flexion (forward motion) and extension (backwards on the way up), so would fit into the sagittal plane.
Frontal plane motion would include leaning from left to right as in sidebends and lateral raises, or perhaps you might picture jumping jacks for a good image of movement along the frontal plane.
Transverse plane motion is the hardest to picture because the plane is horizontal as it divides the top from the bottom, so it’s hard to get our heads around it being a rotating action. The main thing to remember is rotation. An example of a transverse plane exercise would be floor to overhead diagonals with a medicine ball, and a transverse activity might be swinging a golf club.