The cosine rule is a formula to work out the side or an angle on a triangle. Usually you will be provided with two sides and an angle to do this, or the other way around.
The formula to work out the length of a side:
e.g. a^2=b^2+c^2-2bc×Cos(A)
The formula to work out the length of a side:
e.g. Cos(A)=(b^2+c^2-a^2)/2bc
This can be used to find missing angles and sides of a non-right-angle triangle as well as a right-angle triangle.