Wetlands are areas where water is the primary factor controlling the environment and the associated plant and animal life. They occur where the water is at or near the surface of the land, or where the land is covered by shallow water.

The Ramsar Convention takes a broad approach in determining the wetland which comes under its aegis. Under the text of the Convention, wetland are defined as:

"Wetlands are areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporarty, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six metres" (Artcle 1.1). In addition Ramsar sites "may incorporate riparian and coastal zones adjacent to the wetlands, and islands or bodies of marine water deeper than six metres at low tide lying within the wetlands" (Article 2.1).

Five major wetland types are generally recognized:

Marine (Coastal wetlands including coastal lagoons, rocky shores and coral reefs);
Estuarine (including deltas, tidal marshes and mangrove swamps);
Lacustrine (wetlands associated with lakes);
Riverine (wetlands along rivers and stream); and
Palustrine (meaning "marshy" - marshes, swamps and bogs).

In addition, there are human-made wetlands such as fish and shrimp ponds, farm ponds, irrigated agricultural land, salt pans, reservoirs, gravel pits, sewage farms and canals. The Ramsar Convention has adopted a Ramsar Classification of Wetland Type which includes 42 types, grouped into three categories: Marine and Coastal Wetlands, Inland Wetlands and Human - made wetlands.

Wetlands occur everywhere, from the tundra to the tropics. How much of the earth's surface is presently composed of wetlands is not known exactly. The UNEP-World Conservation Monitoring Centre has suggested an estimate of about 570million hectors (5.7 million km2)-roughly 6% of the earth's surfaces - of which 2% are lakes, 30% bogs, 26% fens, 20% swamps and 15% flood plains. Mitsch and Gosselink, in their standard textbook wetlands, 3rd ed. (2000), suggest 4 to 6 % of the earth's land surface. Mangroves cover some 240,000km2 of coastal area, and an estimated 600,000km2 of coral reefs remain worldwide. Nevertheless, a global review of wetland resources prepared for Ramsar COP7 in 1999, while affirming that "It is not possible to provide an acceptable figure of the areal extent of wetlands at a global scale", indicated a best minimum global estimate at between 748 and 778 million hectares. The same report indicated that this minimum could be increased to a total of between 999 and 4,462 million hectares when other sources of information were taken into account.