Hatha yoga is a branch of yoga primarily concerned with mastery of the body, and it is arguably the tradition most familiar to Western culture.
The term of Hatha is derived from the Sanskrit Ha, meaning "Sun," and Tha, meaning "Moon," leading to the common interpretation that Hatha practices are designed to unite and balance these two energies within body. However, the literal translation of Hatha is "force," alluding to the physical techniques used within this tradition.
Hatha yoga can be described as a set of willful and active practices that aim to achieve harmony and balance in the Body, Mind and Energy. This approach to yoga is often called "the forceful path" and includes the practice of physical postures, breathing techniques and some other forceful techniques which goes through gross physical body to subtle Pranic energy.
Hatha yoga is a technique that was developed from theories and techniques of Tantra Yoga. The tantrics embraced the physical body as the means to achieve enlightenment and developed the physical-spiritual connections and body-centered practices that lead to Hatha Yoga. But Hatha Yoga is uniquely focused on transforming the physical body through purification and the cultivation of the subtle energies of the body.
The ultimate goal of Hatha Yoga is to direct the vital energy into the central channel and draw it up towards the crown chakra. Most modern classes do not teach the esoteric exercises of breath retention, bandhas and mudras that would channel and raise one's energy levels high enough to attain enlightenment. And all of the techniques of this form of yoga are seen as preliminary steps to achieving the deeper states of Meditation and enlightenment found in the path of raja yoga.
The oldest and most widely used ancient text on the physical practices of Hatha Yoga is the Hatha Yoga Pradipika. This book was composed in 15th century CE by Swami Swatamarama and is derived from older Sanskrit texts, the teachings from well-known teachers, and from Swatamarama's own yogic experiences. The main goal of this text is to illuminate the physical disciplines and practices of Hatha Yoga and integrate these with the higher spiritual goals of Raja Yoga.
If you see the major practices that are covered in four chapter of Hatha Yoga Pradipika are in six number which are known as six limbs of Hatha Yoga as follow:
Swatamarama begins with explaining the relationship between the two, informing us that Hatha is a preliminary practice for Raja Yoga. He tells us that obtaining self-control and self-discipline is much easier when we start with the physical and energetic body, versus trying to directly control the mind as in Raja Yoga. Through the mastery of the prana, or energy of the body, we can then easily master the control of the mind and obtain success with Raja Yoga.