What is "bhakti?" What does "bhakti-yoga" mean?
"Bhakti means 'devotional service.' " (The Nectar of Devotion [NOD], p xii)
Bhakti connotes devotion, or love, and love implies service. Service refers to that activity which is intended to please the beloved. Thus "devotional service" is an eloquent definition of Bhakti.
Love is the most fundamental drive of every living entity. We cannot be happy without satisfying this desire to love, and we will not be able to perfectly fulfill this desire without permitting its expansion to the most inclusive extent.
"In the primary stage a child loves his parents, then his brothers and sisters, and as he daily grows up he begins to love his family, society, community, country, nation, or even the whole human society. But the loving propensity remains imperfectly fulfilled until we know who is the supreme beloved." (NOD, p. xv)
Love can become all-embracing only when we understand that Krishna is the supreme beloved, the root of all creation. By embracing Him we successfully embrace everyone, just as pouring water on the root of a tree successfully nourishes every leaf and branch.
The Nectar of Devotion teaches us the science of loving every one of the living entities perfectly by the easy method of loving Krishna.
The word yoga means "to connect." Though yogas are commonly used for health and fitness, the Vedic texts explain that yoga is meant to connect with God.
Bhakti means "devotion," or "devotional service," or "pure love." So Bhakti-yoga means to practice connecting with God, and reestablishing our relationship with Him, through acts of love and service, or devotional service.
Bhakti-yoga and Vaishnavism are synonymous terms.