For complete beginners, yoga is simply a series of asanas which improve health and wellbeing in an holistic way. Regular practise of these postures will stretch and strengthen every muscle, use all your joints through their full range of movement, massage your internal organs and improve your circulation, hormonal system and breathing capacity.
Yoga is a procees. As one progresses, breath control (pranayama) is gradually introduced. The physical benefits are enhanced by a sense of calm and contentment.
The word Yoga however means to unite. In practising yoga, one is on the journey to uniting body, mind, intellect and soul. There are in fact eight limbs of yoga. The first two of these, yama (conduct in relation to others) and niyama (personal discipline), are commandments for leading a good life and are, in essence, a pre-requisite for the practice of yoga. The second two limbs are asana (postures) and pranayama (breath control) as mentioned above. The next three limbs, pratyahara (withdrawal of the senses), dharana (concentration) and dhyana (meditation) are not exactly taught, they in some ways happen to you when practising asanas and pranayama matures. The final limb Samadhi being the ultimate goal, a state of total equilibrium.