Applied mathematics is a field of mathematics which uses mathematics to solve problems of other branches of science. There are many fields:
Approximation theory: Sometimes it is not possible to get an exact solution to a problem, because this might take too long, or it may not be possible at all. Approximation theory looks at ways to get a solution which is close to the exact one, and which can be obtained faster.
Probability and Statistics: How likely is it that something will happen? - If a coin is flipped 100 times, and lands heads up 53 times, is this coin good for games of chance, or should another one be taken?