Yoga, often defined simply as “union”, “union with the divine”, or “the way”, and often believed mistakenly to consist only of the popular physical postures called “hatha yoga”, consists of spiritual, mental and physical practices, teachings, and disciplines, that originated in India. There is no one definition that is acceptable to everyone, but perhaps a reasonable general definition is that yoga is a means of achieving the inner realization of the union that already exists (though forgotten and hidden) between individual consciousness and universal consciousness. (In the West, we might call this ‘achieving communion with God’.)
There are many different yogic ‘paths’, each one emphasizing its own techniques and methods, but all of them aiming toward the same goal. (I would suggest that the East has done a far better job than the West in recognizing that different paths are complementary, not enemies). The Bhagavad Gita, the conversation between the god Krishna and the warrior Arjuna that appears in the Mahabharata, speaks of several of these paths:
Jnana Yoga is the process of converting intellectual knowledge into wisdom. (An aside: in the Gospel of John, in my opinion, this is symbolized as “turning water into wine”.) ‘Jnana’ literally means ‘knowledge’, but here we are speaking of the process of meditative awareness that leads to illuminative wisdom: not a method for finding rational answers to external questions (which is what western science is about), but a practice of meditation leading to self-enquiry and self-realisation.
Bhakti Yoga is the process of transforming our emotions and feelings in order to realize the divine nature inherent in every human being. It is often referred to as the yoga of devotion, and it is certainly focused on the Heart, but rather than emphasizing a devotion to an external God, Bhakti Yoga guides the yogin to experience the Unity of all Life.
Karma Yoga emphasizes detachment from the results of our actions, thus leading to greater control of our desires, ambitions, and ego, and all the ways we enslave our souls to the material world. The aim of Karma Yoga is to attain freedom from the bondage of karma through selfless involvement in all activities.
There are, of course, many other yogic paths: Raja Yoga, Hatha Yoga, Pranayama Yoga, Tantric Yoga, Kundalini Yoga, etc.