A keystone species is one whose loss would be particularly damaging to the local ecosystem.
Robert T. Paine introduced this idea in 1969, after an experiment on the coastal ecology on Washington state. He found that the removal of the sea star Pisaster ochraceous caused local biodiversity to plummet; the mussels that the sea star normally ate took over the shoreline, preventing other species (barnacles, seaweeds, limpets, snails, anemones, sponges, etc.) from growing. The term keystone species, and the more specific keystone predator, have since become very important ideas in the field of ecology.
It’s hard to come up with an exact definition of what exactly qualifies a species as a keystone species; very few species can be removed from an environment with out causing changes throughout the entire ecosystem. The usual definition given is some variant of "a species whose removal from the ecosystem would cause greater effects on the ecosystem than that species' biomass would lead you to expect". (This distinguishes 'keystone species' from 'dominant species', those species which play an important part in the ecosystem because they have such a large biomass).
Keystone species are often predators (or heavy grazers), but can also be any species that modifies its environment to any great extent (prairie dogs, beavers trees, and grasses {trees and grasses protect against erosion}), or that produce or provide some limited resource (for example, cacti, which many species use as a water source, and diazotrophic microorganisms, which fix nitrogen). Humans are a keystone species both as a predator (particularly in the past) and as a modifier of the physical environment (particularly in the present).