A Promise Ring is a ring given as an outward expression, or symbol, of a promise made.
Traditionally, the promise ring is given for virtually any type of promise made, either romantic or platonic. Popular reasons to give promise rings include:

• Pre-engagement: If you're sure she's the right one for you, but not quite ready for engagement, the pre-engagement ring expresses commitment to the relationship and a promise for the future together.
• Chastity or abstinence: Often called Purity Rings, these rings are generally given by a parent to a child and symbolize a promise by the child to remain sexually pure until marriage. It is a constant reminder that true love can wait for a sexual relationship. Purity rings can also be given to symbolize a child's pledge to remain substance free.
• Monogamy: In the absence of marriage or pre-engagement, promise rings can be exchanged as a mutual commitment of unity and monogamy.
• Friendship: Good friends have always made pacts, sealing them with the exchange of friendship rings and other small tokens.