Functional testing is not the same as system testing as system testing involves validation of a program in comparison to the published system or user requirements, whereas, functional testing is carried out by checking a program with respect to established specifications and available design documents for the software/applications.The regression testing refers to any type of software testing, which attempts to identify bugs, which are present in either the functional or the non-functional areas of a system subsequent to making modifications such as configuration and patch changes. The key function of regression testing is to ensure that the use of a patch or upgrade does not lead to the introduction of a new bug into the existing system.

Additionally, regression testing helps ensure that the changes in one section of the software do not induce changes in another part of the software's code. Some of the commonly applied regression testing methods include the use of earlier tests to check for alterations in program operation and the search of any previously fixed bugs, which had re-emerged subsequent to introduction of the new code. Fixed bugs in software often re-emerge and regression testing is one of the leading methods to ensure that such re-emergence is identified and easily controlled before any lasting damage occurs. Software development companies repeatedly perform regression testing of software/applications after any change in coding such as use of patches etc. to ensure that the functionality of the application is unimpaired.