Simply put, you give or get an engagement ring after a marriage proposal. A promise ring is much more flexible in meaning and is generally not considered the same as an engagement ring.
Promise rings are gaining popularity among couples who may not feel the need to put a label on their relationship or stick to a timetable. For them, a ring is a symbol for people who move in together and have no immediate plans for marriage but are clearly in a committed relationship. If an enagagment ring comes along in the future, the promise ring often moves over to the opposite hand.
Promise Rings
Flexible in style and budget, a promise ring can realistically be any type of ring you want.