Mangroves extend over 18 million hectares worldwide, covering a quarter of the world's tropical coastline. The largest single system is the 570 000 hectares of the Sundarbans of Bangladesh, which harbour the Bengal tiger and sustain some 300 000 people! Mangroves are, in general, a group of trees and shrubs (of mostly evergreen nature) that have convergently evolved to survive in shallow intertidal environments. Mangroves are defined physiologically as trees that can survive in the mangrove habitat, the mangal. According to Tomlinson (1986), the following criteria must be met for a species to be designated a "true" mangrove species.

1. Complete fidelity to the mangrove environment.

2. Plays a major role in the structure of the community and has the ability to form pure stands.

3. Morphological specialization for adaptation to the habitat.

4. Physiological specialization for adaptation to their habitat.

5. Taxonomic isolation from terrestrial relatives.

Thus, mangrove is a non-taxonomic term used to describe a diverse group of plants that are all adapted to a wet, saline habitat. Mangrove may typically refer to an individual species. Terms such as mangrove community, mangrove ecosystem, mangrove forest, mangrove swamp, and mangal are used interchangeably to describe the entire mangrove community.

The approximately 50 mangrove species that are found worldwide belong to 20 genera in 16 families, although two families, Avicenniaceae and Rhizophoraceae, dominate in number of species and abundance.

Mangroves ecosystems are found in sheltered bays and estuaries as well as along riverine systems influenced by tidal movements. Occasionally mangroves are also found on carbonate platform substrate such as on isolated islands in the Caribbean . Mangroves grow primarily along tropical coastlines but extend as far south as South Africa and Argentina and north along the southern coast of the US and Japan . The main constraint on their latitudinal distribution is temperature wherefore the existence of mangroves typically coincides with the 20°C isotherm.

The mangrove ecosystem is a demanding one. Typically, mangroves are inundated by tides and are therefore in a near-constant waterlogged state. Since the ecosystems are of coastal or estuarine nature the tidal water is saline, so mangrove trees have the added problem of coping with salt and acquiring sufficient water against an osmotic gradient, whilst at low tide, or during heavy rains or floods, they can be exposed to fresh water (low salinity). In very hot climates the saline environment is magnified by evaporation, and in some areas such as the Indus Delta in Pakistan the prevailing salinity can be twice as high as that in seawater. Among the vascular plants, only mangroves can flourish and thrive in such an inhospitable environment.