A computer backup is created to save important information. In case of a system failure due to any reason, the backup enables a quick recovery to the state the system was in when the back up was created.
System backups must be created regularly; usually a partial backup is created whenever a new application is installed so that if there were an error during installation it would be possible to recover by using the back up created.
More extensive back ups involve not just system settings but application installations, personal data, etc. Essentially everything on the hard drive can be backed up as a system image. To ensure this is kept safe in case of a system failure a reliable external hard drive should be used. Redundant backups of different drives can be created to avoid problems in case one of them were to fail though this is not needed in most cases.
Basic software applications for the creation of backups is provided as a utility with all operating systems. Other third party applications can provide a more comprehensive set of options.