First, let me define yoga. Many books have been dedicated to this task alone but, here, I will provide a simple definition.
Historically, yoga is one of the six fundamental systems of Indian thought. Though not religious in nature, yogic philosophy provides tools to achieve spiritual freedom, or, moksha. The primary text and foundation of yoga as we know it today is Patanjali's Yoga Sutras.
Etymologically speaking, yoga has many meanings. The most common meaning is "to unite" or "to come together". In fact, this is where the word yoke comes from.
Hatha is a Sanskrit term comprised of two smaller words: ha, meaning "sun", and, tha, meaning "moon". Therefore, Hatha Yoga translates as "to unite the sun and the moon" (or, "the left and the right"). Essentially, Hatha Yoga seeks to bring opposing forces or energies into balance by the guidelines set forth in The Yoga Sutra.
The essential elements in a physical yoga practice begin with centering and setting an intention for the practice, performing the postures (or, asanas), deep relaxation/integration (or, savasana), breathing and chanting (pranayama and mantra), and, ultimately, meditation.
By performing these steps, one can harmonize competing energies on all levels of their being. However, it is important to remember that this practice is the journey, not the destination, towards a more complete, whole, and authentic life.