A key is a small, low-lying coral island. Like all coral islands, keys are the remnants of ancient coral reefs, and many keys are still ringed by healthy reef ecosystems.

Over time, the top of a coral reef is exposed to the surface. Waves and wind slowly transport sediment, sand, shells, and even living organisms to the exposed reef's depositional node. A depositional node is simply a part of the reef where currents are less likely to erode the material deposited there. Usually, depositional nodes are at the reef's highest elevation.

Keys can provide a rich, diverse ecosystem. The nutrient-rich sand and sediment often support coconut palms and other plants. Sturdy mangrove forests often thrive in the coastal areas of keys. The shallow, shadowed waters of these mangrove forests offer habitats for shorebirds, such as egrets and herons.

Coral reefs that often surround keys provide one of the most biodiverse habitats on the planet. Coral reefs boast an array of fish, sponges, marine mammals, and crustaceans, in addition to the corals that support the ecosystem.

Native animal species in keys are often smaller than their mainland counterparts. Key deer, for instance, are an endangered species native to the Florida Keys. Key deer are related to white-tailed deer, but adapted to their small, isolated environment by evolving to grow less than half the size of their North American cousins. Key deer weigh only 25-25 kilograms (55-75 pounds), while white-tailed deer weigh around 60 to 130 kilograms (130 to 290 pounds).

The soil and sediment that support such diverse, healthy ecosystems can also support agriculture. People have been living on keys for thousands of years. In addition to coconuts, key residents often harvest mangoes, dates, guavas, and other tropical fruits. Most livestock on keys, usually pigs, are introduced species.

Agriculture is not a strong economic activity on keys, however. Fishing and tourism are the keys of key economies. Tropical waters surrounding keys include grouper and sharks, and deeper waters include populations of sailfish, tuna, and other large species.

Keys' white sand beaches and tropical waters make them popular vacation spots. The Florida Keys, Caribbean keys (often called cays) and the keys of Australia's Great Barrier Reef draw millions of tourists every year.

The same elements that make keys so inviting can also make them vulnerable. Thousands of tourists and a growing service-based economy (including hotels, charter boats, and fishing opportunities) can stress the key environment. Overfishing, especially for sport fish such as sailfish, can dwindle local fisheries.

Keys are also at risk from hurricanes, cyclones, and sea level rise. These forces can erode the low-lying islands. Even strong tides can have an impact on key beaches and coastal areas. Small keys in the Caribbean Sea have disappeared entirely—becoming sandbars—after a busy hurricane season.