Promise rings can be worn on any finger of either hand, but are often worn on the left ring finger. This is especially true if the promise ring is used as a pre-engagement ring.
Promise rings have come to symbolize different things. They can be a sign of sexual abstinence, like a purity ring, or can symbolize an exclusive dating relationship. Some people give a promise ring as a symbol of friendship or pre-engagement as well.
The promise ring dates back to the 16th century. A man who could not afford to marry often gave his intended a promise ring as a type of placeholder until he acquired the funds to have the official ceremony.