Hatha Yoga is not a separate form of yoga. It is the physical branch of Raja Yoga (raja means "royal"), the ancient , systematic, and scientific, method of reaching superconsciousness or Self-Realization  Raja Yoga is the overall science under which all the yoga techniques, disciplines, and philosophies fall. Its central technique is the practice of meditation.

The Yoga-Sutras of Patanjali's (Eightfold Path to Enlightenment) is also known as Raja-Yoga. This Sanskrit text is composed of just under 200 aphorisms. The word sutra means literally "thread," and which is used as an aide to knowledge and wisdom through the discipline meditation.
 

The Yoga-Sutra was probably written some time in the second century B.C. The earliest available Sanskrit commentary on it is the Yoga-Bhashya attributed to Sage Vyasa. It was authored in the fifth century B.C. and explains the fundamental meaning of Patanjali's aphorisms.
Beyond a few legends very little is known about either Patanjali or Vyasa. Patanjali believed that each individual is a composite of matter (prakriti) and spirit (purusha). The yoga students goal was to restore the spirit back to absolute purity.