Bog, Swamp, Fen, or Marsh?

How does one clearly define the differences between a bog, a fen, a swamp, and a marsh?
It's not as tricky as you might think. 
Consider each term as its own ecosystem, once you see just how different they are, you'll never mix them up again!:
Bogs are depressions in the Earth that fill with rain and snow, covered with sphagnum moss, the water is quite acidic so only unique plants are equip to live there - here you'll find carnivorous plants which receive their nutrients through the insects they eat instead of the water.
Fens are quite similar to bogs but sometimes have small springs or streams feeding the depression in the earth, as well as water from local runoff, be it nearby mountains, hills, farmland. As the water travels to the fen it picks up nutrients along the way which changes the water from the acidic bog water to a more nutrient rich water. While they both have peat, fens can have a wider diversity of plants growing.
Marshes are wetlands that are fed by rivers, streams, and/or springs that host a great variety of grasses, rushes, and reeds. You'll find a lot of cattails and other smaller forbs growing in these areas that make good food and nesting materials for birds, ducks, muskrats, mink, and beaver!
Swamps are quite similar to marshes - same water sources - but are defined by their ability to host woodier shrubs and water-loving trees, you'll find less of the grasses and reeds here, and many more trees. Swamps make for great nesting grounds for migratory birds.

Now that we know what they are, why are wetlands so important?
Wetlands provide everything from homes and habitats for thousands of birds, fish, amphibians, mammals, and plants, to filter ground water, and curb flooding in times of great rains. The productivity of the ecosystem of a wetland has been compared to that of the rainforest or coral reefs!
 
For more information, check out this link.