In golf, there are two different types of bunkers: sand bunkers and waste bunkers.
The Rules of Golf govern what golfers can and cannot do in sand bunkers and waste bunkers, and not knowing the difference can result in disaster and penalty strokes for a player.
If you're not a golfer, you probably imagine bunkers on a golf course as pretty much any area where there's a bunch of sand. After all, golfers colloquially refer to bunkers as "playing on the beach," so why shouldn't every beach be a bunker?
However, that's not the case.
Sand bunkers vs. waste bunkers
Under the Rules of Golf, sand bunkers are defined as "a specially prepared area of sand, which is often a hollow from which turf or soil was
removed." In other words, a sand bunker is filled with sand, and it's enclosed, and it has a rake. Players are considered to be in a bunker of their ball is fully inside the confines of the bunker surround.
The Rules of Golf consider a bunker a hazard. Golfers aren't allowed to ground their club in a bunker before making contact with the downswing of their attempted shot. If they do, it's a two-stroke penalty for "testing the surface" or for improving their lie.
Prior to 2019, golfers couldn't remove loose impediments in a bunker. However, they now can.
Under the newest version of the Rules of Golf, adopted in 2019, a golfer can declare their ball unplayable in a bunker, but they will be allowed to take relief outside of a bunker in exchange for a two-stroke penalty.
There's a different area, called a waste bunker or waste area, that was generally popularized by course architect Pete Dye. A waste area or waste bunker is an area of the course with elements of a bunker -- sand, crushed shells, loose soil -- that are not prepared, not necessarily surrounded by turf and do not have a rake.
In golf, waste areas or waste bunkers are treated like what's called the "general area," meaning it's just like hitting off turf. Players can ground their club in a waste bunker, including taking practice strokes and testing the soil. However, golfers aren't allowed to use practice swings or shot preparation to move loose impediments -- sand, shells, loose soil -- that are naturally part of the waste bunker. If a player does, then they are considered to have improved their lie and are subject to a two-stroke penalty under the Rules of Golf through Rule 8.1.
When a bunker isn't a bunker
Complicating the sand bunker vs. waste area difference is that tournament committees and golf courses can declare what are sand bunkers as not bunkers, allowing golfers to ground their club and test the surface with practice strokes. They can also declare what would otherwise be considered waste areas as bunkers, meaning the course or tournament could treat any sandy area -- regardless of size, treatment or enclosure -- as a bunker.
That's what happened in the 2010 PGA Championship at Whistling Straits, when Dustin Johnson grounded his club in a sandy area on the final hole of regulation. For that week, the PGA of America declared all sandy areas as bunkers and for players to treat them as such. Since Johnson grounded his club before striking the ball, he was penalized and missed the playoff that was ultimately won by Martin Kaymer.