Raja Yoga is concerned principally with the cultivation of the mind, using meditation, to further one's acquaintance with reality, and finally achieve liberation.
The mind is traditionally conceived as the raja (king) of the body. To make the mind king, the body must be "tamed" through self-discipline, and purified by various means. A good level of overall health and psychological integration must be attained before the deeper aspects of yoga can be pursued. Humans have all sorts of addictions and obsessions and these preclude the attainment of tranquil abiding (meditation). Through restraint and purity in diet and behavior, the mind becomes clear, peaceful and able to focus. This "yoke" of discipline that one puts upon oneself is another meaning of the word yoga.
Every thought, feeling, perception, or memory you may have causes a modification, or ripple, in the mind. It distorts and colors the mental mirror. If you can restrain the mind from forming into modifications, there will be no distortion, and you will experience your true Self.Swami Satchidananda
Patanjali's Yoga Sutras begin with the statement:
Yoga limits the oscillations of the mind. They then detail the ways in which mind can create false mental objects, and advocate meditation on real objects, to produce a spontaneous state of quiet mind, the Nirbija ("seedless" samhadi), in which there is no mental object of focus.
So, Raja Yoga, unlike other forms of Yoga, avoids obsessional practices that can create false mental objects. The aim is samhadi, a state of consciousness in which the experiencing subject becomes one with the experienced object, and in which the mind becomes still (one-pointed or concentrated). The person remains conscious. Sahaj (easy) samadhi is the effortless and continual state of perfection of a satguru (true teacher). Buddhism considers this a precursor for enlightenment, or Nirvana.
(adapted from wikipedia)