Almost all the yoga practiced and taught in the West falls under the category of Hatha (pronounced Hart - Har) Yoga, the yoga that starts with the body. And classical Hatha Yoga is deeply grounded a roughly 2000 year old philosophical treatise by a scholar called Patanjali, referred to as the The Yoga Sutras. This treatise is broken up into four sections called feet, or Pada in Sanskrit, and in the second Pada a path is laid out to applying yoga to point yourself in the direction of a state of personal contentment and equanimity called Samadhi. There are eight limbs (Ashtanga in Sanskrit, and this is from where Ashtanga Vinyasa Yoga derives it's name) or steps in this path, and our Yoga Practices stem from applying these eight limbs.
The most obvious part of a Hatha Yoga class is the Asana, the sanskrit name for the postures of Yoga, but in Patanjali's Yoga Sutras this is actually the third limb. The first two limbs are the Yamas, a list five things that things we should control, and the Niyamas, a list of five things we should encourage. So if the ultimate goal of yoga is cultivating a life of physical, psychological and emotional contentment and equanimity, before we even get on our mats and work through our bodies we need to observe the Yamas and the Niyamas.
One of the things I love most about these eight limbs of yoga, apart from their effectiveness, is their immense practically. The Yamas and Niyamas are all common sense things that you'll be familar with, and they are all things that you have personal control over.
The Yamas (the things we should control)
1. Ahimsa - Do no harm
2. Satya - Be truthful
3. Asteya - Do not steal
4. Brahmacharya - don't over indulge the physical urges (eating, sex, drinking)
5. Aparigraha - Don't lust after material things
The Niyamas (things we should encourage)
1. Soucha - Cleanliness and grooming (Yes, brushing your teeth is a yogic practice)
2. Santośa - Contentment; being satisfied with the present. (ś pronounced 'sh')
3. Tapas - Literally fire: be zealous, be passionate, be active and engaged.
4. Svadhaya - Self study or Self knowledge
5. Iśvara Pranidhara - Surrendering to the ultimate nature of things that you can't change.
So essentially it's very sensible stuff, and we've all been taught versions of principles the Yamas and Niyamas throughout our lives by parents, teachers, and through our own life lessons. That is one of the very special things about Yoga for me: It's not something esoteric or magical, and the elements that make it up aren't unique to the yoga tradition: Yoga pulls together, describes and teaches innately human ways of finding peace and equanimity that otherwise might take us a lifetime, if ever, to stumble upon and find for ourselves.