The origins of yoga lie probably in the Vedic period (1500-500 BC), where we can find stories of ascetic Brahmans that cultivated contemplative techniques of Meditation (called dhyana) and physical self-discipline (tapas). This kind of ascetism became especially popular during the sramana movement, a philosophical phenom that gave birth to the religions of Buddhism, Jainism and other sects that don't exist anymore. Indeed, it is around this time that the term yoga appears, described in the Upanishads as a mean to withdraw the senses into a spiritual state, and the Mahabharata follows by introducing multiple kinds of yogas. By the time Alexander the Great arrived to India, he would find something we can already identify as primitive yogis: ascetics that liked to remain motionless in different postures, reportedly hardening their bodies so their minds would be strengthened as well.
The first regularization of yoga was done by the sage Patanjali, whose Yoga Sutras synthesize the yoga traditions of multiple Hindu schools in a eight-part system named ashtanga, basically an instructional of meditation and a code of spiritual behavior. During the Middle Ages, after influencing and being influenced by Buddhist and Jain schools, yoga received a push in its physical aspect (called hatha yoga) with the rise of tantra, esoteric rituals that help advancing towards enlightenment through a variety of edgy practices (yes, including the famous tantric sex). It is then that yoga started increasingly emphasizing asanas or postures, not only to sit in meditation, but also by their own value: 84 postures are constantly mentioned in texts, although usually not described. The end of this pre-modern yoga came in the 19th century, the moment in which the art started being influenced by European physical culture through British contact.
The modern evolution of yoga was moved by nationalism, as Indian yoga teachers absorbed all the gymnastic culture they could find, both native and European, in order to counter the British racial stereotype of Indians being genetically decadent weaklings. Most modern non-seated asana were integrated at this point, as well as the Surya namaskar or sun salutation sequence, originally an unrelated exercise practiced by Indian wrestlers. Modern yoga schools would soon appear, with masters like Vivekananda, Shri Yogendra, Kuvalayananda and Paramansa Yogananda becoming avid travelers in order to disseminate their art in the west, where first oriental-inclined people received it with enthusiasm before it eventually found its way into mainstream western culture. Due to some religious discomfort, it was (and still is) usually marketed as a form of exercise and relaxation, without any Hindu connotation, but never without this vibe of ancient, fathomless mystic art.
Especially in United States, yoga has become strongly associated with New Age believers, hippies, cults, vegans and office ladies preoccupied with their weight, but there is always much more underneath.
Warning: it might be due to the cultlike environment of some Indian schools, or maybe the lack of regulation in some areas, but if you navigate through the history of modern yoga, you will find that a startling number of important masters in the past have featured scandals of sexual abuse, racism, homophobia or discrimination of some kind, so this is definitely a discipline where one is forced to separate the art and the artist in order to achieve that inner peace they preach about.
There are lots of yoga styles and schools. With the popularization of different yoga methods, there are also modern schools that integrate practices of more than one stylem in any case, these are the most likely to find in your nearest gym.
- Ashtanga Vinyasa Yoga or Msore style: a style created by K. Pattabhi Jois. Its main characteristic is its emphasis on vinyasa, meaning transition, in which the user transitions from one posture to other in a flowing movement, a bit similar to tai chi.
- Sivananda Yoga: a system founded by Vishnudevananda. This is a very dedicated, almost monkish yoga style, where practitioners are encouraged to keep an old-fashioned Hindu lifestyle.
- Integral Yoga: an allrounder style, so to speak, founded by Satchidananda. It is a very adaptable, complete system integrating postures, meditation and the like. Note that there are also schools that call themselves "integral" on the same principle but are unrelated to this one.
- Bikram Yoga or Hot Yoga: the school of Bikram Choudhury. Bikram Yoga uses only 26 postures with few variations, and has the interesting (and grueling) custom of keeping the place of practice hot in order to simulate the humid climate of India.
- Iyengar Yoga: created by B. K. R. Iyengar, this is a style based around precision and balance that tries to reach the best alignement possible in every posture. Iyengar popularized the usage of belts and foam blocks as aids for the physically limited to perform postures without danger of injury.
- Yin Yoga or Taoist Yoga: a mix of yoga and traditional Chinese medicine, innovated by martial artist Paulie Zink as a complement to other styles. Most of its masters teach soft, non-straining variations of postures with a strong meditative focus.
- Power Yoga: a hardcore, fitness-crazy system that descends from the Ashtanga Vinyasa. Most of the yogi performing spectacular one-arm handstands you often find in internet pics (and other similar stunts) come from this school.
- DDP Yoga: a modern form created by professional wrestler Diamond Dallas Page, who presented his style as a "yoga for regular guys".
