Disk[x, y, r] is a two-dimensional graphics primitive that represents a filled disk of radius r centered at the point x, y.
Disk[x, y, , ] yields an elliptical disk with semi-axes and .
Disk[x, y, r, , ] represents a segment of a disk.
Angles are measured in radians counterclockwise from the positive direction.
Disk[x, y, , , , ] yields an elliptical disk segment obtained by transforming a circular disk segment with the specified starting and ending angles.
Scaled and Offset can be used in the radius specification (see notes for Circle).
See The Mathematica Book: Section 2.9.2 and Section 2.9.2.
See also: Circle, Polygon.