Generally, wetlands are defined by various characteristics, such as soil type, level and duration of saturation throughout the year, and the types of resident plant and animal communities are dominant factors. Wetland types vary due to regional differences in soil, topography, climate, hydrology, water chemistry, vegetation, and level of human disturbance. Most wetlands fall into four main types: marshes, fens, bogs, and swamps.

For regulatory purposes under the Clean Water Act (CWA), wetlands are defined as "those areas that are inundated or saturated by surface or ground water at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs and similar areas."

In New York, the Department of Environmental Conservation (DEC) describes wetland areas as "transitional areas between aquatic and upland plant and animal communities, and often have some of the qualities of both. Wetlands also occur where the groundwater occurs near or at the surface, saturating the soil and the root zone of the plants that grow there." More lengthy legal definitions of "tidal" and "freshwater" wetlands are contained in Environmental Conservation Law (ECL) sections 24-0107.1 and 25-0103.1. DEC has paraphrased the definition of freshwater wetlands as "those areas of land and water that support a preponderance of characteristic wetlands plants that out-compete upland plants because of the presence of wetlands hydrology (such as prolonged flooding) or hydric (wet) soils." Similarly, a typical tidal wetland "is the salt marsh which occurs in the near shore areas all around Long Island, the lower Hudson River, and along the entire Atlantic coast of the United States. These areas are dominated by grasses and other marsh plants which are adapted to the rise and fall of the tide and the salty water it brings."