This word "Bhakti" is a sanskrit word. It is derived from the verb "Bhaj" to this is added a "pratyaya" "kitana" and as a derivation it become "Bhakti". The meaning of the verb "Bhaj" is to offer seva. "Kitana" means with affection. Hence the combined word "Bhakti" means to serve Sri Thakurji with an emotion of love or attachment under Sanskrit grammer when a verb (dhatu) is associated with a "pratyaya" the meaning becomes more leaning on the "pratyaya" than on its verb. Hence with this word "Bhakti" the meaning of "Sneha purvak" i.e., with attachment remained the important factor.
If you offer service (seva) without theaccompanying "sneha" or love it cannot be said "Bhakti". In order to put pressure on the aspect of "sneha" the adjective "Prema Lakshana" is added before the word "Bhakti". Hence the new concept turned out to be "Prema Lakshana Bhakti".
Hence, Bhakti means: attachment towards divine being (Bhagavan). In other words : to serve divine being with love. Since the combined word consists of a verb "Bhaj" it indicates to the acting side of word viz., to serve. If a person shows only attachment towards divine being but does not serve HIM, then we cannot say that he is having Bhakti. Supposing another person attends to the part of seva only - just for duty's sake, or for sustenance or for his stomach's sake, but does not entertain any attachment for divine being then also his seva cannot be called Bhakti.
Let us understand this through an illustration : A women delivers a child in a Maternity Home. A nurse trained in a child care attends to the child. In spite of the special knowledge she attends the child care as part of Nursing home duties so we cannot say that she is doing it with Bhakti. Neither can we say it is seva. Look at the mother. She becomes a mother first time. She has no knowledge of how to care for a child. But she shows immense love for the baby child. Whatever she does for the care of her infant is called "Santana Bhakti". It is seva to Santana i.e., her own child.
Hence it is essential that seva should be enveloped with the sentiment of oneness, attachment. When this love takes the form of action it completes as Bhakti.