Bhakti is an expression of love, devotion and faith centered upon the Supreme Person rather than the Supreme Abstraction. It became a popular folk movement which was very strongly opposed by the Brahmins as it disregarded traditional Vedic rituals, ignored caste differences and placed devotion over knowledge. The Bhagavad Gita is the first major expression of Bhakti which, over the centuries spread throughout the country through the wandering holy men and sadhus. It became the religion of the great masses of India for it enabled the individual to approach the Divine directly and become part of His all-encompassing love.
One of the underlying themes of Bhakti is that of incarnation (avatara), God manifesting himself upon the earth in some form (animal or human) in Order to aid mankind in times of trouble. Krishna is the supreme example as the manifestation of Vishnu.