A promise ring can be given as a sign of friendship, chastity, pre-engagement or simply as a sign that you and your partner are dating exclusively.
The best way to wear the promise ring is to place it on your right ring finger so as to avoid confusion with the engagement ring or wedding band. You can also wear it on the middle finger of your left or right hand or in any finger you want as long as it is not the left ring finger.
Just like a wedding band, a promise ring can be worn at all times and can only mean that you are taken. Some people also use their promise ring as their wedding ring.