Ring guards and ring wraps are used to help keep the actual metal band of a ring from being damaged or broken during daily wear. There are a number of different styles and types of ring guards and wraps, and you'll have to choose the one that best fits your specific ring. Additionally you'll have to make sure that your ring is properly sized to handle a ring guard.
A ring guard is a small sleeve of metal that fits around the actual band of a ring. A guard will leave a very small strip of the inside of the band exposed, but it will cover the outside of the band and protect the edges of the stone.
A ring wrap is similar to a ring guard, but it covers the entirety of a ring rather than all but a single strip. A ring wrap is slipped around the band by a jeweler, often in two separate pieces. This is a much better method for those who want to cover the entirety of their ring rather than just a portion.
Ring guards and ring wraps are priced similarly; it's the material the guard or wrap is made out of that changes the price. Pure gold or white gold will run in the several hundred dollar range, but guards that are made of platinum or other more expensive materials will, of course, cost more. Often they can run up to $1,000 dollars or more.
Ring guards provide slightly less protection than a ring wrap because of the small strip that's left open to the wearer's skin. The outside of the ring should be fully protected regardless of whether you use a guard or a wrap, however.
Protecting the actual band of the ring is one of the most common reasons for buying either a ring guard or a ring wrap. They also can dress up a less expensive ring, or reinforce a fragile or weak ring with an extra layer of metal.