Originally Posted by taoist
"The son's "thread" -- a strip of thin string worn for life, and slung diagonally across his chest and back, from shoulder to waist -- showed that he was also a Brahmin, Hinduism's highest caste. "
The "Sacred thread" is typically white cotton. Bachelors have three strings tied/twirled together. Husbands have 6. Husbands who have lost their father have 9 (this differs from sect to sect).
It is changed every year - typically during the Hindu new year day (naturally, differs from the Gregorian and Julian calenders). It is used as part of every day prayer ritual - a very loose analogy can be drawn to rosary beads.
This thread is also called "Poonal" or "Jan-ae-yo".
The "sacred thread" however, is different from the one work around the waist. This waist thread is typically of a darker hue. While the sacred thread has religious hue - this waist thread has no religious specification... just cultural.
Like Rep pointed out, many asians use them to ward off the "evil eye" (same reasons babies are adorned with "beauty spots").
cheers,