I find this to be a very clear and visually stimulating definition of Hatha Yoga:
"Hatha yoga refers to a set of physical exercises (known as asanas or postures), and sequences of asanas, designed to align your skin, muscles, and bones. The postures are also designed to open the many channels of the body—especially the main channel, the spine—so that energy can flow freely.
Hatha is also translated as ha meaning "sun" and tha meaning "moon." This refers to the balance of masculine aspects—active, hot, sun—and feminine aspects—receptive, cool, moon—within all of us. Hatha yoga is a path toward creating balance and uniting opposites. In our physical bodies we develop a balance of strength and flexibility. We also learn to balance our effort and surrender in each pose.
Hatha yoga is a powerful tool for self-transformation. It asks us to bring our attention to our breath, which helps us to still the fluctuations of the mind and be more present in the unfolding of each moment."
Cindy Lee, "Yoga Questions Answered" (Yoga Journal Online)