Exotic is a classification used for a race of intelligent alien which does not in one or many ways conform to the norm. Most common are:
1) They live in environments toxic to most other races (Izlians)
2) They have bizarre or nonstandard physiology. This can include non-carbon based or biology wildly contrary to accepted norm. (Depik)
3) Their psychology is not of a defined order, as in their actions or responses to external stimuli are not predictable by other sapient species. This may also extend to no visible sign of intelligence, but evidence suggests that they are, in fact, sapient (Wrogul)
Some exotic races include more than one of these factors.