Error: Error is a human action that produces an incorrect result. Error means normally arises in software Error means to change the functionality of the program. This could be happen because of confusion in understand requirement. Bug: Bugs arise from mistakes and errors, made by people, in either a program’s source code or its design. Simply Bug is an error found BEFORE the application goes into production. Defect: It is the difference between expected and actual result in the context of testing. Defect is the deviation of the customer requirement. Failure:It is the inability of a system or a component to perform its required functions within specified performance requirements. Failure occurs when fault executes.