Kirtan is a form of Bhakti Yoga, the yoga of devotion. Kirtan means devotional chanting, usually the singing of ancient Sanskrit chants and mantras in a group setting. Here is a wonderful description of kirtan written by Krishna Das:

 "The words of these chants are called the divine names and they come from a place that's deeper than our hearts and our thoughts, deeper than the mind. And so as we sing them they turn us towards ourselves, into ourselves. They bring us in, and as we offer ourselves into the experience, the experience changes us. These chants have no meaning other than the experience that we have by doing them. They come from the Hindu tradition, but it's not about being a Hindu, or believing anything in advance. It's just about doing it, and experiencing. Nothing to join, you just sit down and sing."