The Promise Ring can be mistaken as an engagement ring because both signify love and commitment but a promise ring is that - a promise or an understanding about each other's commitment, whereas an engagement ring depicts the progression of the love and relationship into the next level, which is marriage.
The Promise Ring is totally not new. In fact, the millennials have embraced this type of symbolism to depict a promise that is to be kept true and honoured by the receiving party. Oftentimes, it is given as an exchange ring to the other party in the relationship or whomever the promise ring is presented to. It can be customized or made into personalized promise ring, engraved with the giver and recipient's name like the picture below.
(Photo credit: jowjeweler.com)
This type of ring can mean many thing. Although, it is commonly used by lovers, even parents can present a different form of a promise ring to their child to symbolize the promise of being pure, as it is called Purity Promise Ring.
What ever the promise ring is used for, it shows the symbol of love and commitment entered to by both the giver and the recipient.