Hatha is a style of yoga which develops both flexibility and strength, among many other benefits. It includes a balance of breathing practices, physical postures and relaxation. The asanas are done relatively slowly and deeply, rather than moving fast from one to the next. 

Many classes are only one hour long - most of my mine are 90 minutes long which means that the pace in unhurried, to have some headspace and time out, and to experience each practice more deeply. This is also true of the relaxation as there is proper time for this in the session not just a brief 2 or 5 minutes. Our physiology changes for the better when we relax so this is a key and integral part of the yoga.