Radial creates a radial gradient. If an exact circle is required then the wh (width / height) button should be pressed in the area parameter. These should then be edited so that they are identical. A hard-edged circle can be had by setting the softness paramet to 0.
There isn't much else to say about Radial other than that it does its job beautifully. It is very useful for masking off a color adjustment and its softness parameter can be edited without compromising its edge values too much. I frequently use it to mask out nebulous regions of an effect. It is faster to use and to process that a Roto or RotoPaint node.