Upanayana is one of the traditional Saṃskāra (rites of passage), that marked the acceptance of a student by a Guru (teacher) and an individual’s entrance to a school in Hinduism. The tradition is widely discussed in ancient Sanskrit texts of India, and varies regionally.

The Upanayana was restricted, in many medieval Indian texts to upper three of four varnas (caste, class) of society, including Brahmin, Kshtriya and Vaishya. However, older Vedic era texts such as the Baudhayana Gryha Sutra encouraged all members of society to undertake Upanayana rite of passage, including the Sudra. Women were encouraged to undergo Upanayana rite of passage in ancient India, either before they started Vedic studies or before their wedding.

In Hindu traditions, a human being is born at least twice – one at physical birth through mother’s womb, and second at intellectual birth through teacher’s care, the first is marked through Jatakarman sanskara ritual, the second is marked through Upanayanam or Vidyarambha rites of passage. A sacred thread was given by the teacher during the initiation to school ceremony, and was a symbolic reminder to the student of his purpose at school as well as a social marker of the student as someone who has been born a second time he went about collecting fire wood in forest and food donations from villages on a daily basis.

Source: http://en.wikipedia.org/wiki/Upanayana