A sadhu is a religious ascetic, mendicant (monk) or any holy person in Hinduism and Jainism who has renounced the worldly life. It literally means one who practices sadhana or keenly follows a path of spiritual discipline. Although the vast majority of sādhus are yogis, not all yogis are sādhus.

The sādhu is solely dedicated to achieving mokṣa (liberation), the fourth and final asśrama (stage of life), through meditation and contemplation of Brahman. Sādhus often wear simple clothing, such as saffron-colored clothing in Hinduism, white or nothing in Jainism, symbolizing their sannyāsa (renunciation of worldly possessions).

The processes and rituals of becoming a sadhu vary with sect; in almost all sects, a sadhu is initiated by a guru, who bestows upon the initiate a new name, as well as a mantra, (or sacred sound or phrase), which is generally known only to the sadhu and the guru and may be repeated by the initiate as part of meditative practice.

Becoming a sadhu is a path followed by millions. It is supposed to be the fourth phase in a Hindu's life, after studies, being a father and a pilgrim, but for most, it is not a practical option.

Source: http://www.britannica.com/EBchecked/topic/515896/sadhu-and-swami; Klaus K. Klostermaier (2007). A Survey of Hinduism: Third Edition. State University of New York Press. p. 299.