Whether it be the kachera (of sikhism), the sudreh and kusti (of zoroastrianism), or the tallit katan (of orthodox judaism), sacred attire connected to vows or obligations is a common element in many spiritual traditions. Hinduism is the same. Men of the brahmin, kshatriya, and vaishya castes frequently wear a "sacred" thread (or janeu), which they wear over their left shoulder and across to their right hip. This practice dates back to ancient India and serves to remind men of their sacred obligations to their families, their gods, and themselves.
There are four castes or sects in india: the brahmins, the kshatriyas, the vaishyas, and the shudras. Originally, the brahmins were those who sought spiritual knowledge; however, today, anyone born to a brahmin is considered one—which is ironic, since you wouldn't consider the son of a lawyer a lawyer just because his father was a lawyer.
The sacred thread ceremony is now typically performed by brahmins when they are eight years old. However, the kshatriyas and vaishyas, the following two castes, typically hold this ceremony and get the janeu when they are getting married. (Some Brahmins will delay their participation in the janeu rite until they are getting married, but this is less frequent among Brahmins.) Kshatriyas and vaishyas typically wait until the day or perhaps the week before their wedding to take on the thread. Like a brahim, they don't perform all the hindu ceremonies. Additionally, kshatriyas and vaishyas frequently stop wearing the janeu as soon as they get married.
The term "sutra" in Sanskrit refers to the thread we use. The word "sutra" literally means "thread," and some of the most revered hindu writings are known as "sutras" because they "weave" knowledge like a thread that binds things together.