Hatha yoga is another school of yoga like classical Ashtanga Yoga. Even though Hatha Yoga and Ashtanga Yoga is same from some point of yogic principle, they are known as two different school of yoga from yogic present point of view.
In Hatha Yoga “Ha” means “hot”, sun, pingola, yang, positive, Sympathetic nervous system. “Tha” means “cool”, moon, ida, yin, negative, parasympathetic nervous system.
You can see this hatha yoga concepts from micro particle to macro particle. Atom is stable when photon & electron is equal. No species if there is no male & female sex. Union and equilibrium state is important ot the harmony. When prana flows with uniform motion in both ida & pingala, it merges with king of the nadi’s sushuma(spine). This is the concept of the “HATHA YOGA”.
The text Hatha Yoga Pradeepika has mentioned the following shloka in constrant with my expalantion.
‘Ha’karh keerthitha suryathi
‘Tha’karh Chandra uchyathe
Surya chandrama suryoogath
According to classical text it is said that Hatha Yoga originally masterd by lord shiva(Adinatha). Later it is transferred to the yogi mastyendra through a beautiful mythological story. (I have not explained here).Lord shiva appointed him to spread yoga to the entire earth to light on divinity finally it is taught to the “swathmarama” the contibuter of hatha yoga Pradeepika Hatha Yoga pradeepika has 389 shlokas deals with method of asona practice, secret of pranayama explanation of Mudra’s and details of Rajayoga.