Isotonic exercise takes the body, or the part of the body being exercised, through a range of motion while under resistance. An example of isotonic exercise would be walking, running, hitting a bag, or using a barbell in a movement like a bench press or a squat, all good for maintaining bone density and building strength and endurance.
Isometric exercise requires no motion, as the muscles and joints stay in a fixed position while under resistance. The yoga plank, wall squat, and held poses in yoga are examples of isometric exercises; they are easy on the joints and good for maintaining and, to some degree, building strength, maintaining bone density, and lowering blood pressure (after elevating it).
In this video, Richard uses examples of both isotonic and isometric exercise while performing a seated chest press.
These principles can be applied to any exercise.