Peatlands are distinguished from other wetlands, such as marshes and swamps, by the buildup of large amounts of partially-degraded plant material called peat. This material can accumulate to several meters depth in bogs, and essentially all the peat in Northern wetlands has built up since the receding of ice-age glaciers about 10,000 years ago.

Bogs (Gaelic for soft ground) are dominated by the moss species Sphagnum, which forms a soft mat at the top, and will also contain sedges and shrubs in the family Ericaceae, and pitcher plants. The major source of water to bogs is rainwater, which contains only trace amounts of mineral nutrients. Sphagnum spp. have the ability to grow in environments with very low concentrations of mineral nutrients. Dead Sphagnum plant material absorbs water and mineral nutrients, releases acids that cause the pH to drop to 4 or below, and degrades only slowly under waterlogged anaerobic conditions. These factors allow the buildup of large amounts of peat that impedes the flow of mineral nutrient-bearing groundwater into the bog. Thus, the conditions in the bogs favor Sphagnum and a few other plants adapted to low nutrient conditions, such as pitcher plants that get their nutrients from the insect prey they trap.

McLean Bog, dominated by a mat of Sphagnum and sedges and shrubs.

Fens (Icelandic for quagmire) are dominated by sedges (such as cattails) and grasses and receive nutrient-rich groundwater from surrounding areas. The pH in sedges is usually near neutral and peat buildup is typically less than two meters. 

Michigan Hollow, a fen dominated by Carex spp.