A wet area of the earth's surface characterised by an abundance of specific bog vegetation and the presence of a peat-forming process. There are lowland and transitional bogs, which are fed by groundwater, and upland bogs, which are mainly fed by precipitation. Sometimes the formation of a bog is linked to the accumulation of groundwater in the surface layers. The usual herbaceous vegetation gives way to wet plants such as mosses, sedges and cottongrass, which die and form peat. The largest bog on the planet is the Amazon floodplain.

Wetlands are formed in two main ways: by waterlogging of the soil or by the overgrowth of water bodies. Swamps can be caused by human activity, such as the building of dikes and dams for ponds and reservoirs. It is also sometimes caused by beavers.

Wetlands play an important role in the formation of rivers. Wetlands prevent the development of the greenhouse effect. Like forests, they can be called the "lungs of the planet". In fact, the reaction of the formation of organic matter from carbon dioxide and water during photosynthesis is opposite in its overall equation to the reaction of the oxidation of organic matter during respiration, so that when organic matter decomposes, carbon dioxide bound by plants is released back into the atmosphere (mainly through bacterial respiration). One of the main processes capable of reducing the carbon dioxide content of the atmosphere is the burial of undecomposed organic matter, which occurs in peat bogs, forming deposits of peat, which is then converted into coal. Draining peat bogs, as was done in the 19th and 20th centuries, is therefore ecologically destructive. On the other hand, peatlands are one of the sources of bacterial methane (a greenhouse gas) in the atmosphere. In the near future, the amount of peat methane in the atmosphere is expected to increase due to the melting of permafrost. Peatlands are natural water filters and sanitarians of agro-ecosystems.

Depending on the conditions of water and mineral nutrition, bogs are divided into

Lowland (eutrophic) - a type of marsh with rich water-mineral nutrition, mainly due to groundwater. They are located in river floodplains, on the shores of lakes, in places where springs come out, in low places.

Transitional (mesotrophic) bogs are between lowland and upland bogs in terms of vegetation and moderate mineral nutrition.

Oligotrophic (oligotrophic) bogs - usually located in shallow watersheds, fed only by rainfall, with very little mineral content, their water is extremely acidic.

Wooded - covered with low pine, heathland shrubs, peat moss.

Ridge and bog - similar to woodland, but covered with peat bog and with few trees.

In general, a distinction is made according to the type of vegetation that predominates: forest, shrub, grass and marshland.

Micro-relief type: hilly, flat, convex, etc.

Macro-relief type: valley, floodplain, slope, watershed, etc.

According to climate type: subarctic (in permafrost areas); temperate (most bogs in Russia, the Baltic States, the CIS and the EU); tropical and subtropical. Tropical wetlands include the Okavango Swamp in South Africa and the Parana Swamp in South America. Climate determines the flora and fauna of wetlands.