Promise rings are typically inexpensive bands exchanged between young couples that have pledged exclusivity in their relationship. While they can be thought of as pre-engagement rings, promise rings might fall more along the lines of exchanging class rings, or a commitment to "go steady." Additionally, they can symbolize any promise or pledge made between people, even friends. For this reason, they are also referred to as friendship rings.
In the case of romantic relationships, promise rings remind each bearer of the monogamous link to his or her partner. Once they are exchanged, both people assume they can rely on the promise of the other to remain true. This often ushers in the first serious relationship between young people.
Though there are no formal rules or traditions regarding these rings, generally speaking the more expensive the ring is, the more serious the promise. Girls that receive promise rings with expensive gems might very well expect these to be precursors to engagement rings. There is nothing more awkward than giving a heartfelt gift only to have more read into it than intended. Therefore, make the promise clear when exchanging rings.
Friends might exchange promise rings for any number of reasons, from sealing a vow, to making a wish, to simply celebrating the bond of friendship. These rings are a great way to make any commitment real, and keep it foremost in mind. Though these are traditional reasons to exchange promise rings, a ring is a nice reminder for nearly any significant promise — including a promise made to yourself. Even a moderately expensive gold or titanium band or pinky ring can be a constant daily reminder of the investment you are making in yourself to meet a desired goal too long put off, rise to a difficult challenge, or fulfill a lifelong dream.
Like the eternal circle that has no beginning or end, promise rings speak of infinity and bonds that cannot be broken. That which is bound to a ring 'benefits' from the symbolism and beauty of the ring, which serves to strengthen the bond or promise. Promise rings are a beautiful way to remind ourselves of the allegiance we feel to those people and things that mean the most to us.