The appendix is not used by our body. It is thought that once it used to digest grasses, but as we evolved we did not need to use it.
The body does need the appendix and functions fine without it. Having said that sometimes the appendix can get inflamed causing a lot of illness and pain. If the appendix ruptures, urgent medical attention is required for the immediate removal of the ruptured appendix before the poison spreads throughout the body causing the person to die due from septicemia (bacterial poisoning).
Here is a definition; it's a body part attached to your lower intestine.
http://digestive.niddk.nih.gov/ddiseases/pubs/appendicitis/