The above is a Circle Menu using the Ellipse interaction mechanism. The mouse is over the "Active Desktop" menu option. Accordingly, the pie slice that corresponds to that option is lit up, informing the user of exactly how far it extends. This helps a user learn where they should point in order to select a particular menu option. The pie slices for the various menu options are not all the same size because the top and bottom options take up more space around the circle (visually), they are allotted larger slices. The size of a menu option's slice varies with the number of degrees it takes up around the circle.
For more information about pie slices, look
at the Pie Menu interaction mechanism.