To find your inseam, lay a pair of well-fitting trousers flat and measure along the inner leg from the crotch seam to the hem. That number, combined with your waist, tells you a jeans size in the waist×inseam format common to denim brands.
Inseam controls where the hem lands and how the jeans "break" over your shoe. Too long and the fabric pools at the ankle; too short and it rides up when you sit — which is why tall and petite ranges exist mainly to adjust inseam.