Question

What's the difference between isometric/isotonic exercises?

Answer 1

In isotonic contractions, the muscle contracts and shortens, giving movement. Nearly all the training you do is isotonic.
where as in isometric contractions, the muscle contracts but does not shorten, giving no movement. The Plank is a good example of an isometric contraction

Answer 2

isometric contraction: muscle is not moving, but there is tension
isotonic contraction: muscle length shortening, and there is tension