A yoga class described as ‘Hatha’ will typically involve a set of physical postures (yoga poses) and breathing techniques.
Esoterically 'ha' represents the sun and 'tha' represents the moon. Literally however, hatha means ‘force’ so although postures are practised more slowly with more static holds than say vinyasa flow or ashtanga, hatha yoga is traditionally defined as ‘the yoga of force’, or ‘the means of attaining a state of yoga through force’.
So, hatha yoga can be considered as anything you might do with the body, including:
asana - yoga postures
pranayama - breathwork
mantra - chanting
mudra - (hand) gestures
shatkriyas and shatkarmas - cleansing techniques