Width is the distance between the ends of the bar, generally measured centre to centre but some manufactures measure outside to outside just to make it difficult.

Reach is the distance forward from the horizontal part (ie the area the stem clamps) to the drops.

Drop is from the horizontal part to the level of the drops.

Again drop and reach are generally measured centre to centre but not always.

(Some text lifted from Sheldon Brown cause I'm too lazy to type).