An exotic species is a species that historically lives in one place, but that has been moved to a new geographic location due to human interference. There are other terms for an "exotic" species including non-native, non-indigenous and alien species.
Sometimes exotic species are created when humans accidentally take a species and move it geographically, such as some species of rats, which were accidentally brought over on ships. However, many times the movement is done deliberately, such as when starlings were brought over from Europe to North America.
Exotic species do not necessarily have to harm the environment, and can even benefit it. Exotic species are most harmful when they compete for resources directly with another species in the geographic location, and are much better at competing that the native species cannot survive.