The most significant difference between a sand trap and a bunker is in its design. A sand trap is a man-made pit on the course that is then filled with sand. A bunker is also a depression on the course (either natural or man made), but it doesn't always have to be filled with sand. It could be filled with pine needles, long grass, dirt, gravel, sand or many other things. Because of this, bunkers are also known as (unless filled with sand) waste areas or waste bunkers.
Since sand traps are man-made, they are strategically positioned in areas on the course that make the game more difficult. A shallow depression filled with dirt and pine needles sitting 200 yards away from the green is a bunker. A sand-filled pit pin-pointed just a few yards from the hole is a sand trap.
A sand trap is considered a hazard and there are several rules that must be followed when hitting your ball from a hazard. One of the best known rules is called grounding. When your ball lands in a hazard, you're not allowed to let your club touch the ground before or during your swing. You will also see a rake next to the sand trap, and you're expected to rake the trap after playing your stroke. If your ball lands in a bunker that isn't filled with sand, you don't have to follow the hazard rules.
Playing Bunker and Sand-Trap Shots
When your ball lands in the sand, you don't have many club options to choose from. You'll likely have to use a wedge club, and a sand wedge is made specifically for this instance. If your ball lands in a bunker, you can be a little more flexible in your club choice. Depending on what the bunker is filled with, you may be able to use any club in your bag.