Life Jackets are commonly referred to as life vests or PFDs (personal floatation device). Always look for the US Coast Guard approval and don't paddle without one, it's "Kayaking 101".

A USCG Type III PFD is used for kayaking. They are not the bulky apparatus they used to be, the waist is cut higher for paddling comfort. They are lightweight, comfortable and come in some snazzy colors.