Circle[x, y, r] is a two-dimensional graphics primitive that represents a circle of radius r centered at the point x, y.
Circle[x, y, , ] yields an ellipse with semi-axes and .
Circle[x, y, r, , ] represents a circular arc.
Angles are measured in radians counterclockwise from the positive direction.
Circle[x, y, , , , ] yields a segment of an ellipse obtained by transforming a circular arc with the specified starting and ending angles.
Scaled[, ] or Scaled[, , , ] can be used in the radius specification. The are in scaled coordinates, and the are in ordinary coordinates.
Offset[, ] can be used to specify radii in printer's points.
The thickness of the circle can be specified using the Thickness primitive.
See The Mathematica Book: Section 2.9.2 and Section 2.9.2.
See also: Disk.