To those of us in the west, Raja Yoga brings us into more familiar territory, yoga-wise. What we refer to as yoga is one part of Raja Yoga. The yoga poses we think of as yoga are called asana. The primary purpose of asana is to make meditation easier and it is, in fact, a moving meditation. The primary goal of yoga is to master the mind.
The basic premise of raja yoga is that our perception of the divine Self is obscured by the disturbances of the mind. If the mind can be made still and pure, the Self will automatically, instantaneously, shine forth.
How? To quote the Bhagavad Gita:
Patiently, little by little, spiritual aspirants must free themselves from all mental distractions, with the aid of the intelligent will. They must fix their minds upon the Atman, and never think of anything else. No matter where the restless and unquiet mind wanders, it must be drawn back and made to submit to the Atman alone.
Patanjai and The Yoga Sutras
Below is a commentary of the Yoga Sutras and Raja Yoga by Swami Vivekananda
Don't even get me started on Swami Vivekananda. In my opinion, one of the most brilliant people who ever lived.