Energy can be said to exist in, among other forms, two basic forms: potential and kinetic. Potential energy is essentially stored energy: energy which exists within the position of elements of a system. Through work, this stored or potential energy may then be transformed into kinetic energy, or the energy which exists within an object that is in motion.
Potential energy exists within an object or a system that is trying to return that object to some original or simpler position. For example, a mass which is high off the ground – because it has been lifted, for example – is constantly pulled by gravity back toward the surface. In the same way, a spring which is pulled and stretched will attempt to recoil itself back into an unstretched position. In both cases, the energy needed to move them into these positions is stored within the object; it has become potential energy. It can then be released from the object in the form of kinetic energy, moving the object into another position.
Different types of energy imply different types of potential energy: for example, gravitational potential energy (in the first example), elastic potential energy (in the second example), and also other forms of potential energy, such as electrical and nuclear potential energy, also exist. Most commonly, however, in basic physics a discussion of potential energy will involve a discussion of gravity. In this case, what is measured is the change in energy levels (i.e. the energy stored) when an object is lifted off the ground, which is equal to the mass multiplied by the acceleration due to gravity (9.8 metres per second squared, on Earth) times the change in height. (Note that in physics, mass and weight are technically different concepts.)
In imperial measurements (as are used in the United States, for example), potential energy is measured in calories. However, in metric measurements (used throughout most of the rest of the world), potential energy is measured in joules.
Kinetic energy, in contrast, is the energy of an object in motion – or, put in terms of basic physics, the work which accelerates that object from relative rest (i.e. not moving) to a particular velocity. (Note that in physics, speed and velocity are different concepts.) The object in question will retain that level of kinetic energy so long as it retains that velocity; if the velocity increases, its kinetic energy will increase, but if the velocity decreases, its kinetic energy will decrease.
For example, a car will use energy stored in chemical form (in gasoline or diesel fuel) to accelerate to a particular speed, in the process converting that chemical energy into the kinetic energy of motion. Once the car is travelling at that speed, the only further work required is to overcome inefficiencies due to friction and air resistance. In most models used in basic physics classes, friction and air resistance are assumed to be negligible. This is rarely true in real life, but occasionally can be so: for example, a spacecraft, like a car, uses chemical energy to obtain a particular speed, but once it is in orbit, it no longer encounters air resistance and therefore retains in full the kinetic energy it gained during liftoff.
Based on theoretical work by Leibniz and Bernoulli, kinetic energy is equal to the mass times the square of the velocity of the object in question. (Note that Einstein’s theory of relativity complicates all such energy questions somewhat, but not usually until students have advanced beyond basic physics.) Like potential energy, kinetic energy is measured in imperial calories, and metric joules.