This system is based on a very practical, direct instruction, which was written down in short, poignant sentences called sutras - known as the Yoga Sutras, by Patanjali, a (maybe-) contemporary of Buddha.
Patanjali gives a psychological and philosophical explanation of what is commonly known as 'the human mind'. What it can reach, how to get there, what problems you might encounter on the way and how to avoid or overcome them.
Patanjali used the word 'Ashtanga' (ashta = 8, anga = limb), the 8 limbs of the yoga system :
- Yamas (five of these. Wise acting/relating to your environment)
- Niyamas (also 5. Living wisely or 5 characteristics of someone who does)
- Asana (Posture / Exercise)
- Pranayama (Breath & energy - regulation)
- Pratyahara (Understanding the senses / often simply translated as 'withdrawal of the senses')
- Dharana ('concentration')
- Dhyana ('meditation')
- Samadhi ('Enlightenment')
Without (for the moment) diving further into these - yoga is a system. That is, it has a beginning, a path with directions and itself (the state of yoga) as a goal. It starts like this :- You do detailed, sometimes difficult exercises in connection with deep and easy (not strained) breathing, you move, work, stretch and open the whole body. All together with a strong focus/awareness on breathing,body, how & what you feel and an intense training of the mind and perception. Then, with the union of all these comes the possibility for yoga, the dissolution of the single elements.
In other words : The practice of yogasana & pranayama is possible only in conjunction with pratyahara, dharana, as well as the yamas and niyamas. Only then can they (and the yogi) unite to become one ('meditation') and form the basis for samadhi. The last 2 elements (dharana & dhyana) are nothing to practice, but 'simply' happen on the fertile ground of the previous limbs. They can not be attained but come as a present.
Patanjali's Yoga Sutras are very often seen as rules. But Patanjali doesn't set any rules to be followed. He simply explains the stepping stones of yoga.
It's obvious that the above does not even scratch the mirror of the surface or this topic, we recommend some reading material here.
The part of Yogic Philosophy, that concernes our actions in life, and within that our vision / defeinition of ourselves (i.e. the 'yamas & niyamas') can be found nicely laid out in Donna Farhis first book, as a basic intro to Yoga.
A very good translation of the Yoga Sutras by Godfrey Devereux's can be found in the links in this same category.
Any other philosophical 'rantings' must not necessarily have the same mentioned quality, but this shall be decided by the readers thereof.