An arc is part of a circle defined by a center point and two end points. In gCADPlus, as shown in the figure below, you have a number of different options for placing an arc and these are applied by different fly-out options.
Tip: Osnaps are usually used to control placement of start, endpoint and center of arcs.
Handles can be used to adjust the start and end points (and diameter of the circle from which the arc is taken after an arc has been inserted.
There are other entities that are built from arcs - circle, circular arc, ellipse, and elliptical arc. Each of these subtypes is an arc with slightly differing properties.