An instinct is any innate behavior i.e. any behavior that an organism performs without any prior experience or learning. Fixed action patterns are a subset of instincts. The behavior is relatively short, does not show much variation between members of the species, is performed in response to a sign stimulus, and is usually continued until completion. The example given on Wikipedia is the retrieval of eggs by certain geese. If an egg is moved within the nest, the goose will move the egg back. The moved egg is the sign stimulus, and the behavior is continued to completion even if the egg is removed. The goose will move its beak like the egg is still there. It can't help finishing its movement. An example of an instinct that is not a fixed action pattern would be something like seasonal migration. The behavior is long, and it's not like you could show a bird a picture of winter or change the temperature to get it to perform the very long migration (there is no one sign stimulus) .
