This is a very informative discussion about Yoga. Yoga is the most interesting, complex and scientific system in Hinduism, particularly Raja Yoga. Agnideva and Vajhradara have already explained Yoga so well, I will only add towards it.
There are many paths of Yoga, each that would suit different dispositions and personality typs. They all lead to the same goal, that is
union with the supreme soul and with ultimate reality. In Patanjali's Yoga Sutras, the states of existence are delinated as the gross, the subtle and the casual, each of which are formed of finer and finer atoms. The casual is the ultimate and primordial reality.
This ultimate state can be attained through these various paths of Yoga, including the physical yogas, namely Hatha Yoga and Kundalini Yoga. Hatha Yoga is not just an exercise regimen, it is based on the model of the body being an energy system. The Yoga Sutras postulate that the human body is composed of a physical, subtle(astral) and casual body and they are interrelated. In the astral body there are thousands of energy centres or points called
chakras and they are connected through networks of spiritual channels called
Nadis These regulate the flow of prana(life force) around the body. There are thousands of minor chakras and
7 main chakras situated on the spine that govern specific aspects of our psychological, emotional and physical health:
The above image shows the 7 main Chakras in the human body.
Through Hatha Yoga, the Hatha Yogi works on his chakras and Nadis to improve the spiritual health of the astral body by clearing up
blockages and facilitating the flow of Prana in the body. The various postures(asanas) and locks(mudras) and breath control exercises(Pranayama) are designed specifically to work on this.
Kundalini Yoga is based on the same model, however here the emphasis is on awakening a dormant spiritual energy called Kundalini at the base of the spine(coiled up) which is discussed in the Hatha Yoga Pradipika and is often depicted as a coiled up snake with it's tail in it's mouth. Through Kundalini Yoga exercise, which also include postures, locks and breath control, but are targeted on the spine, this Kundalini energy is made to ascend the spinal column towards the crown chakra leading to the state of superconsciousness.
Laya Yoga, while not a physical Yoga, functions on the same model. Here the Laya Yogi will meditate on the Chakras directly. The Chakras are seen as doorways into consciousness and by meditating directly on them one can experience them in their fullness e.g. The heart Chakra governments the emotions of love and by meditating on it one can attain infinite and unconditional love.
Jnana Yoga, the yoga of knowledge is based on the main principle of
Neti Neti, which means "Not This, Not this" through self inquiry and study a Jnana Yogi tries to find the answers to who he really is and what true reality is by eliminating everything that it isn't and discriminating between illustory and transistory and the real and the absolute.
Then there is Bhakti Yoga - the yoga of devoition, which is for somebody with a religious disposition. The main belief in Yoga and Hinduism is that supreme soul pervades all and is omnipresent. In other words if we have eyes to see god, then we can see him in anyone, in anything. It is about forming a loving relationship with god and with people and about cultivating unconditional love.
The final and highest form of Yoga is Raja Yoga -the most sophisticated and scientific form of Yoga. It is also called "Royal science" This path is for those of a more scientific and intellectual disposition and enlightenment can only be attained through pure logic and mental concentration.
Again to reiterate, though these paths are completely different in techniques, they all lead to the same goal. That is Yoga.