We just introduced one form of energy kinetic energy, but there is another equally important form: potential energy.
We'll talk about it now, but first I'll let you know a good way to think about it. You can think of it as stored energy, that could be converted to kinetic energy to do work. An example of this is a waterfall. The water at the top of the fall has a fairly low kinetic energy but a lot of potential energy. By the time it gets to the bottom of the fall, it has lost a lot of its potential energy and has gained a lot of kinetic energy.
Potential energy is useful in dealing with so called conservative forces. In this case one can derive conservation of mechanical energy. That is, the sum of kinetic and potential energy is conserved.