Error : A discrepancy between a computed, observed, or measured value or condition and the true, specified, or theoretically correct value or condition. See: anomaly, bug, defect, exception, and fault
Failure: The inability of a system or component to perform its required functions within specified performance requirements. See: bug, crash, exception, fault.
Bug: A fault in a program which causes the program to perform in an unintended or unanticipated manner. See: anomaly, defect, error, exception, fault.