Hatha Yoga was orginally only concerned with balancing the function of our breath, there were originally only very few postures that were practiced, these postures were primaarily concerned with the movement of pranic energy. Over time more postures were introduced that took out the impurities from the body opening up the body to allow the pranic current to be liberated.
Ha-tha literally means Sun and Moon, it refers to the very essence of Yoga which is to balance to two major energy channels or nadis. The right channel , the sun channel (pingala) starts with the right nostril and the left channel or moon channel (ida) begins in the left nostril. have you ever thought about why you have two nostrils?
Hatha yoga has now become a universal term for yoga postures and breathing, but the subject is very vast and involves other practices such as mudras and bandhas.