Raja Yoga is designed for persons of an experimental nature. The experiment being conducted is upon oneself; the hypothesis: that the human self is composed of three bodies, the Physical body, the Astral body, and the Causal body. In order to verify this hypothesis, and experience the subtle components of oneself, the raja yogi follows a system of concentration and mental control known as the Eightfold Path. Most of the popular styles of Yoga are forms or derivations of Raja Yoga (eg: Hatha, Vinyasa, Ashtanga, Kundalini).
Physical Body (Annamaya Kosha; "Food Sheath")
This is the visible component of the human body, the portion that changes most noticeably over the course of one's lifetime.
This is the energetic portion of the human self, and passes vital energy to the physical body. When the connection between the Physical and Astral bodies is broken, the Astral body departs and the Physical body dies. This component is itself composed of three layers:
Pranic Sheath (Pranamaya Kosha): the etheric double of the physical body; it is composed of 72,000 nadis (similar to the meridians of Traditional Chinese Medicine) through which vital energy (prana or, equivalently, chi) flows, and a multitude of chakras (where many nadis come together) where vital energy pools.
Mental Sheath (Manamaya Kosha): the seat of the five senses; it controls the automatic functions of our daily lives and is restless by nature, as it is constantly receiving input from the senses.
Intellectual Sheath (Vijnamaya Kosha): home of the ego, the "I-sense"; this sheath makes decisions and interprets the world based on the input it receives from the mental sheath.
Causal Body (Karana Sharira; "Seed Body")
This body contains one's Karma. The subtle impressions of this karma control the formation and growth of the other two bodies, and determine every aspect of the next birth. This body has only one layer, the Anandamya Kosha (Bliss Sheath), in which reside the experiences of happiness and joy. The Astral and Causal bodies remain together at the time of death, leaving behind the physical body.