Hatha Yoga and Raja Yoga, though similar in many aspects, are different from each other in respect to their points of emphasis. While Hatha Yoga focuses itself nearly completely on the perfection of the body, Raja Yoga, though without denying the importance of purified, robust body, trains its focus on the mind.
Raja Yoga believes that mind can be freed from its slavery to the body and by achieving this freedom, it can get immersed in the Supreme Consciousness. Although Raja Yoga accepts asanas and pranayamas it is not as much concerned with them as hatha yoga is.
If we study the functioning of our mind as a witness we will understand that it is an exaggeration to call man a mental being as the human mind is mostly entangled in the physical/nervous system. Human mind is so much entangled in the mechanical operations of the physical/nervous system of the body that science and psychology refuse to believe that there is anything like pure mental substance. As a matter of fact, human mind is so little developed that it seems to lie at par with the animal mind.
However, against this scientific canon, yoga has been affirming for millennia of time that the mind as an independent entity can exist for itself and if humans so wish the mind can be freed from its entanglement in body or physical/nervous system and then it can assert its own independent reality as pure mental substance unsullied by physical consciousness.
This desire to release the mind from its entanglement in the body consciousness led to a detailed inner study and it culminated in the emergence of Raja Yoga- the classical school of yoga that is meant to free the mind from its slavery to the body.