Bug: A bug can be defined as the initiation of error or a problem due to which fault, failure, incident or an anomaly occurs.
Error: A human action that produces an incorrect result.
Fault: An incorrect step, process, or data definition in a computer program.
Failure: A failure is said to occur whenever the external behavior of a system does not conform to that prescribed in the system specification. A software fault becomes a software failure only when it is activated.