A Bhajan or kirtan is a Hindu devotional song, often of ancient origin.
Traditionally, the music has been Indian classical music, which is based on ragas and tala (rhythmic beat patterns) accompanied by voice and traditional Indian instruments.
Great importance is attributed to the singing of bhajans with Bhakti, i.e. loving devotion, by which we feel closer to either our true self (jiva, atma), or to the Universal God.

Can the devotional songs we sing in our homes make a difference to the world?
What bliss is there in bhajans! What a demonstration of oneness is it when a myriad throats join in uttering the name of God! The vibrations emanating from them make the heart vibrant. If you sing alone in your shrine, the vibrations return to you as a reaction. But in community singing, what you have is not a reaction but a wave of vibrations. They enter into the atmosphere and purify the polluted air. The atmosphere today is polluted by bad thoughts and feelings. When you sing the glory of God, the bad germs in the air are destroyed and the air gets purified by a treatment of anti-biotics, as it were.

The devotional songs you sing, saturate the atmosphere with thoughts of God, and of the peace and joy that He showers. The songs affect not only the particular area you are and the environment, but the entire atmosphere. Continue the attitude of devotion and humility, of service and tolerance, and the atmosphere will not be polluted by hatred. Do not contaminate the air by voices of acrimony, scandal, insult or slander. Keep silent when you feel like expressing such ideas, that itself is a service to yourself and to others.

- Sri Sathya Sai Baba Discourse, Nov 14, 1976. & Jan 26, 1995