The word Yoga comes from the Sanskrit root 'yuj' which means 'to join'. Yoga is the science that teaches us the method of uniting the individual soul with the Supreme Soul. Yoga is an exact science, a study of human body and mind. Yoga is not acrobatics, magic, or torture. Lastly, Yoga is not a religious cult.
Patanjali formulated this science into a definite system under the name of Ashtanga Yoga or Raja Yoga. Raja Yoga is the king of Yogas. The Yogi seats at ease, watches his mind and silences the bubbling thoughts.
The First Chapter is Samadhi-pada. It deals with different kinds of Samadhi. It contains 51 Sutras. Obstacles in meditation, five kinds of Vritti and their control, three kinds of Vairagya, nature of Ishvara, various methods to enter into Samadhi and the way to acquire peace of mind by developing virtues are described here.
The Second Chapter is Sadhana-pada. It contains 55 Sutras. It treats of Kriya Yoga, viz., Tapas, study and self-surrender to God, the five Kleshas or afflictions, the methods to destroy these afflictions which stand in the way of getting Samadhi, Yama and Niyama and their fruits, practice of Asana and its benefits, Pratyahara and its advantage, etc.
The Third Chapter is Vibhuti-pada. It contains 56 Sutras. It treats of Dharana, Dhyana and various kinds of Samyama on external objects, mind, internal Chakras and on several objects, to acquire various Siddhis.
The Fourth Chapter is Kaivalya-pada or Independence. It contains 34 Sutras. It treats of the independence of a full-blown Yogi who has perfect discrimination between Prakriti and Purusha, and who has separated himself from the three Guna. It also deals with mind and its nature. Dharmamegha Samadhi also is described here.
Obstacles in Meditation
Patanjali says: "Disease, dullness, doubt, carelessness, laziness, worldly-mindedness, illusion, missing the point, unstability - these are obstacles in Yoga." 
Yama or Eternal Vows: 
Ahimsa (non-violence) 
Satya (truth) 
Asteya (non-stealing) 
Brahmacharya (continence) and 
Aparigraha (non-avariciousness); 
Niyama or Observances: 
Saucha (purity) 
Santosha (contentment) 
Tapas (austerities) 
Svadhyaya (study) and 
Ishvarapranidhana (surrender to God); 
Asana (firm, comfortable meditative posture); 
Pranayama (the regulation of the Vital Force); 
Pratyahara (abstraction of the senses and mind from objects); 
Dharana (concentration); 
Dhyana (meditation); and 
Samadhi (superconscious state or trance) 


There are thousands of manuals on yoga, including classical traditions of yoga, particularly hatha yoga. Postures and other practices like Pranayama (breath control), and dharana is gaining popularity. There are many yogic traditions in different parts of the world under different names. Just listing few links.