Lockback Knives (also referred to as a back lock) have a single blade and have a locking mechanism that locks the blade in the open postion.  This lock is positioned In the back of the handle and consists of the tang swiveling a spring loaded back that locks into place once the blade is in the open position.  These type of knives are inherently strong in that the spine and handle once locked nearly form a full tang type of fixed blade.  To close the blade the lockback is pushed down which released the blade for folding / storage.