What it isn't?
A lot of people misinterpret Yogic Science as a bunch of people bending themselves into weird positions which is only one aspect of it. When westerners think if Yoga they it is a dicipline of yoga named: Hatha_Yoga.
Scriptures
Free Hatha Yoga Pradipika (.pdf) Ebooks (offers Bhagavad Gita, Katha Upanishad, Samkhya Karika and Yoga Sutras)
Gurus
Sivanada audio &
transcript
Related:
Meditation
Breath
Nadis