used in modern graphical user interfaces
to represent one of a group of mutually exclusive
Radio buttons are usually circular in shape, so as to distinguish them visually from checkboxes, which are square. Radio buttons are always found in groups of two or more, because they represent a choice between options -- not merely the enabling or disabling of an option. A group of connected radio buttons is usually set apart from other interface elements by a space, or sometimes by a divider line of some sort, so that the user can tell which options are affected by the radio button selection and which are not.
The distinguishing attribute of a group of radio buttons is that when you select one of them (usually by clicking the mouse on it), the other buttons in the group become unselected. This enforces the mutual exclusivity of the options the buttons represent. In most GUIs, the selected button becomes dark, or gains a black or colored spot in the middle; the others stay or become light.
Radio buttons, like many other interface elements, can be greyed out when the options they represent do not make sense, or are unavailable for other reasons.
The name "radio button" comes from the consoles of old radios, which featured similar buttons to select mutually exclusive options, such as AM/FM or just power on/off. These buttons were cylindrical or round in shape, and when the user pressed one button in, a lever would pop the other buttons in the group back out.