Mantra is the sequence of words written in Sanskrit (Sanskrit is the language of ancient India), Mantra is verbal formula which is used in Indian Granthas. It use in Prayar and Aarti.
Hindu, Buddhist, Tibitain and other religion or sub religion of Hindu. There can be Mantra for Yoga, Meditation, Prayer and Aarti. OM (picture on your right) is the one of the smallest Mnatra with biggest effect in Hindu religion.