Yoga is a vast subject, not confined to physical asanas or breathing techniques alone. There are many recognised and valid paths to achieving the union signified by the term "yoga", traditionally accepted as alternate paths to the same destination.
Additionally, the recent century has seen a slow but definite revival of interest in yoga, leading to many different schools of yoga being established by specific individuals in their own style. All of these schools derive from one or more of the traditional paths, just presented and practised with the distinctive stamp of its originator.
All yoga can be divided into two groups - transformation either from out to in, or in to out; from the tangible/external to the intangible/internal. Hatha yoga starts with control over the body (external) and moves to control over the mind (internal), while Raja yoga starts with control over the mind (internal) and moves outward to the body.