Apple's GarageBand contains scores of great software instruments and hundreds of audio loops. What it doesn't have is a sampler'"a way for you to create your own instruments from audio files. Since GarageBand lets you use Audio Unit instruments, you can make use of samplers like Native Instruments Kontakt from within GarageBand. But, there's an easy way to make a simple sampled instrument without resorting to a third-party tool.
A perfect application for this is to create a 'sound effect'