Vector graphics are one of the two main types of computer graphics. Unlike bitmaps, vector graphics define a drawing as a series of mathematical values. They can be scaled to any size without any loss of resolution.
While it is impossible to draw a circle with Bézier curves, it is quite possible to emulate one so well that, when viewed by human eyes, the drawing is indistinguishable from an actual circle. All you need to do is use the right value of the Bézier control points. This value is the radius of the circle multiplied by a constant known as the kappa. This book tells you the right value of the kappa.