Circuit training is basically a large range of exercises
aimed at improving an athletes strength. It differs from weight
training in that, typically, the only resistance that is used is the
athletes own body weight, as opposed to providing resistance using
weights machines or free weights. Occasionally a light weight may be used
for specific exercises but this would typically involve many more repetitions
than in normal weight training. Dumbbells can be particularly useful in
this respect and also have the benefit that they are cheap to buy and
easy to store.
An individual exercise may use a specific range of muscles or perhaps
just one muscle isolating this from the rest of the body to provide
improved strength in this area. An example of this could be a calf raise
where only the calf muscle is being used.