The simplest is made of a single patch that is placed below the nocking point between the fingers and the string.
The tab may also have a platform that is attached to the back of the tab and forms a flat cover over the top, to give the archer a hard reference point underneath the chin when the string is drawn back.
Platform tabs are more common for recurve or Olympic-class target archery.
The Japanese yugake is a reinforced glove with a special ridge which holds the string.
The term "tab" is of uncertain etymology, perhaps an alteration of tag (small hanging piece).