A bhajan or kirtan is a Hindu devotional song, often of ancient origin. Great importance is attributed to the singing of bhajans with Bhakti, i.e. loving devotion. “Rasanam Lakshanam Bhajanam” means the act by which we feel more closer to our inner self or God, is a bhajan. Acts which are done for the God is called bhajan.
Kirtans are deeply rooted in Vedic tradition. Bhajans are often simple songs in lyrical language expressing emotions of love for the Divine, whether for a single God/Goddess, or any number of divinities. Many bhajans feature several names and aspects of the chosen deity, especially in the case of Hindu sahasranamas, which list a divinity’s 1008 names.
Traditionally, the music has been Indian classical music, which is based on ragas and tala (rhythmic beat patterns) played on the Veena (or Been), Sarangi Venu (flute), Mridanga(or Tabla) (traditional Indian instruments). The Sikh Scripture contains 31 ragas and 17 talas which form the basis for kirtan music compositions.
Source :http://en.wikipedia.org/wiki/Bhajan