Consider a circle whose centre is at the point (a,b) and whose radius is r:
The equation of the circle is given by
The values of a, b and r are specified whereas x and y are left as general points. This is much like when we specify m and c in y=mx+c when we have the equation of a given straight line.
You may be presented with questions that expect you to know the equation of a circle. Other questions might bring in knowledge from other areas of maths such as finding mid-points. In more complicated questions you may even be asked to find gradients using the knowledge that a tangent to a circle is perpendicular to its radius.