Hatha yoga...did you know? The term "hatha yoga" refers to the practice of the physical asanas (postures) of yoga. So, ALL the yoga teachers and yoga studios are offering "hatha" yoga. Under that broad category are the specific varieties of hatha yoga, such as Iyengar, Bikram, Anusara, Viniyoga, Vinyasa, Kundalini...
Then there are the other kinds of yoga, such as Bhatki yoga, the path of devotion; Karma yoga, the path of action; Raja yoga, Tantra yoga and more.... there's many kinds of other yogas besides the physical pose ("hatha") yoga.
Hatha yoga, the physical poses, are designed to prepare a person for meditation and pranayama (breathing practices). If you've ever tried sitting for meditation, you know how difficult and uncomfortable it can be to simply sit still. Hatha yoga prepares the body for this.
What I'm reading now: "The Anatomy of Hatha Yoga: a Manual for Students, Teachers and Practitioners" by H. David Coulter. It's a dense book and over 600 pages! Required reading for the yoga teacher training I'm taking.