Hatha Yoga describes any of the physical practices of yoga. Remember that yoga has eight limbs, only one of which, asana, involves doing yoga poses. When you do Iyengar, this is hatha yoga; when you do Astanga, this is hatha yoga too. “Hatha yoga” can be used interchangeably with “yoga”.
A class described as Hatha, will be a gentle stretching class with breathing exercises.