Made of paper, muslin or an artificial mesh and designed to hold sufficient tea for one cup. Often there is a thread attached with a label (giving the name of the tea or the company) so the teabag can be removed when the brew is of the requisite strength.
Teabags come in a variety of shapes - notably square/oblong, circular and pyramids, each of which has its benefits. They are usually sold in packets, sometimes as selections so that customers can try different varieties. The teabags may be wrapped in little paper envelopes (and generally are for selections) - these latter have been used for craft purposes (tea bag folding).
A UK Tea Council article on tea bags can be found here .