From the Wikipedia,
Peat (turf) is an accumulation of partially decayed vegetation or organic matter that is unique to natural areas called peatlands or mires. The peatland ecosystem is the most efficient carbon sink on the planet because peatland plants capture the CO2 which is naturally released from the peat maintaining an equilibrium. In natural peatlands the “annual rate of biomass production is greater than the rate of decomposition” but it takes “thousands of years for peatlands to develop the deposits of 1.5 to 2.3 m, which is the average depth of the boreal peatlands.” One of the most common components is Sphagnum moss, although many other plants can contribute. Soils that contain mostly peat are known as a histosol. Peat forms in wetland conditions, where flooding obstructs flows of oxygen from the atmosphere, slowing rates of decomposition.
Peatlands, also known as mires,[Notes 1] particularly bogs, are the most important source of peat, but other less common wetland types also deposit peat, including fens, pocosins, and peat swamp forests. Other words for lands dominated by peat include moors, or muskegs. Landscapes covered in peat also have specific kinds of plants, particularly Sphagnum moss, Ericaceous shrubs, and sedges (see bog for more information on this aspect of peat). Since organic matter accumulates over thousands of years, peat deposits also provide records of past vegetation and climates stored in plant remains, particularly pollen. Hence they allow humans to reconstruct past environments and changes in human land use.