A polarizing filter is one of those nifty devices that help block haze, reduce light glare and increase contrast in clouds. Its difficult to describe. Some say its like having a set of vertical blinds that you can close part way, blocking out the glare on a scene. A polarizer works because of the way light waves travel.
Polarizers are great for shooting water or glass because they block those annoying reflections that plague photographers. There are two kinds of polarizing filters, linear and circular. Circular polarizers are generally compatible with all sensor types on all cameras. This is why they are more recommended than the linear type.
You can expect to loose any where from 1 to 2 steps of exposure when using a polarizing filter, but the effect is worth it's weight. The use of a polarizer can be faked (poorly) in computer during post-processing. However, it takes a great deal of time and patience. Indeed, it is a lot more convenient to have a polarizer in the field than it is to fiddle for an hour or so on your computer.
A tip for your snapshots: I wouldn't recommend it for submission on Dreamstime, but in a pinch, some hi-grade sunglasses can be held in front of your lens to fake the effect of a polarizer. Just remember, the cheaper the sunglasses, the worse it's going to look. This cheap trick is also awkward and usually results in some serious vignetting.
Polarizers are just one of those "must have" filters that are worth an investment.
If you want to read more about the science behind polarized light, check here.