Bust/Chest
Measure from just under the armpit to the armpit on the other side.
Waist
This refers to the natural waist: the thinnest part of your mid-section. So for a dress, the thinnest part is the waistband area. For pants or skirts, this refers to the top of the garment.
Hip
Measure the widest part of a skirt or pants, below the waistband.
Length
For tops and dresses, measure from the top of the shoulder to the bottom of the garment. For pants and skirts, this is the measurement from the waist to the bottom.
Inseam
This is a measurement for pants, and refers to the length from the crotch to bottom of pant leg.