As stated below, the tang is the part of a sword blade which is inside of the handle. Here are some examples of the several types of tangs:

Full Tang: The whole handle is the tang, with handle scales attached to the sides.

Half Tang: Half of the handle is the tang.

Push Tang: The tang is pushed into the handle and is fixed in place.

Encapsulated Tang: The handle is molded around the tang.

Rat-Tail Tang: The blade is secured to the handle by a bolt or threaded pommel.