Bog, marsh, swamp, fen.
Often these words are used interchangeably, but in reality, each is its own type of wetland, which is a word used to refer to water-saturated landscapes. (Watch: What is a wetland?)
A marsh is a wetland that is continually full of water. If you have been to the Florida Everglades, then you have been to a marsh. These areas have soft-stemmed vegetation growing out of the water, because these specific types of plants thrive in soil that is completely saturated with water.
A swamp is a type of wetland filled with woody plants. The soil is saturated during the growing season and has standing water other times of the year.
Bogs have spongy, peat-moss deposits. They have very acidic water, which is not good for plant growth, and they receive most of their water from precipitation --- rain, sleet or snow --- rather than from runoff from the landscape, groundwater or streams emptying into the bog.
Fens are peatlands and are fed by groundwater, which makes them less acidic than bogs and more nutrient-rich. They are home to sedges, grasses and rushes, because these plants love constantly saturated soil but not a flooded habitat. Over time, the peat in the fen builds up, and they feel almost bouncy when you walk on them.