Correction
It is quite possible that customer may find out some defects while working on
the system. Corrective maintenance changes the software to correct defects.
Adaptation
Over time, the original environment for which
the software was developed is likely to change. Adaptive maintenance changes the
software to make it compatible with the current external environment.
Enhancement
While using the software, customer may
identify additional functions that will provide more benefit. Perfective
maintenance extends the software beyond original functional requirements.
Prevention
Computer software deteriorates due to change
and because of this, preventive maintenance must be conducted to enable the
software to serve the needs of its end users. Preventive maintenance makes
changes to computer programs so that they can be more easily corrected, adapted
and enhanced.