Obviously, water hazards are things you want to avoid on the golf course. Hitting into one usually means a lost ball, and always means a 1-stroke penalty (unless you try to hit your ball out of the water, which is not a good idea). Sometimes golf course designers put a water hazard in a position where the only option is to hit over it. And sometimes water hazards run alongside the fairway or to the side of a green (these are called "lateral water hazards").
As with putting greens and bunkers, the size and shape of water hazards vary greatly. Some are natural elements, such as streams. Many golf course ponds and lakes are manmade, however, and so are shaped as the golf course designer wants them. These manmade bodies of water are often more than just cosmetic, too, with many of them serving as catchments for rainwater, holding water for later irrigation use around the golf course.
As noted, the rules distinguish between water hazards and lateral water hazards. Lateral water hazards run alongside the line of play, "regular" water hazards are everything else. But if you can't tell the difference, look for colored stakes or painted lines around the boundary of the water: Yellow means water hazard, red means lateral water hazard. (If you hit into one, the procedure for continuing play is slightly different depending on the type of water hazard.)
Also note that something classified by the golf course as a water hazard doesn't necessarily have to have water in it! A creek might be a water hazard even if the creek has run dry. (Look for those colored stakes or lines. And such features are often noted on the scorecard.)
And those are the major elements that make up a golf course.
Related:
The meaning of colored stakes and lines on golf courses
