Backup software is used to create exact copies of files, databases, or the entire software content of a computer, which may be used later to restore the original content in the event of data loss or corruption.

Backups are a form of disaster recovery, but not all backup systems are able to reconstitute a computer system or database server. Many backup systems are used to recover data only, not the operating system or installed software.

Although the idea behind a backup system is to allow for recovery after a system crash or lesser data loss. However, rarely is recovery a simple matter, regardless of which backup system is in use. There is so much that can go wrong that it is all but inevitable that something will.

There are several types of backup systems or methods of recovery, some more effective than others.

The most basic method might involve manually copying important data onto tapes, DVD-Rs, or external hard drives, or automating the process through the use of a script. This is an easy method to implement, but it is not likely to have a high level of recovery. The data might be there, but getting it back where it belongs, and integrating it, may prove to be very difficult, if not impossible.

System imaging is often used by computer technicians to record known working configurations, but it is not one of the more effective means of storing ongoing backups of diverse systems.

With incremental backups, any data that has been changed since a reference point in time is stored, but duplicate copies of unchanged data are not copied. Typically, implementation of this method will begin with a full backup of all files, after which only changed data is stored. Subsequently, a number of incremental backups are made in accordance with a predetermined schedule. Restoration begins with the last full backup, then continues through the incrementals. Data that has been changed since the last scheduled incremental backup may be lost.

Continuous data protection (CDP) refers to a backup system that instantly saves a copy of every change that is made to the data. This allows restoration of data to any point in time and is the most advanced data protection system.

Often marketed as CDP, near-CDP backup applications automatically take incremental backups at a specific interval, such as every fifteen minutes or once a day. With this system, any changes since the last incremental backup will be lost.

In a reverse incremental backup, the first run of a backup job creates a full backup. The data is copied, block by block, compressed at an appropriate compression level, and then stored in a full backup file. All subsequent backups are incremental, in that only those data blocks that have been changed since the last job are copied. During a reverse incremental backup, any changes are reflected in the full backup file to rebuild it to the most recent state of the full backup. Therefore, the most recent restoration point is always a full backup and gets updated after every backup cycle.

A differential backup only saves the data that has been changed since the last backup. A maximum of two backups from the repository is used to restore the data. Restoration requires starting from the most recent full backup, then applying only the last differential backup.

Whatever the model that is used, the data has to be copied onto a storage medium.

For many years, magnetic tape was the most commonly used medium, and it is still used for large amounts of data. Some tape formats are proprietary or specific to certain markets, such as a particular brand of PC or a mainframe.

As hard drives have become less expensive and able to hold larger amounts of data, the use of hard drives as a backup medium has increased. However, just as the hard drive in your computer is subject to corruption or data, so might the hard drive that your backup data is stored on. Some manufacturers offer rugged portable hard drives that include a shock-absorbing case around the hard disk. Overall, however, the stability of hard drive backups is shorter than that realized by tape backups.

Optical storage devices, such as recordable CDs, DVDs, and Blu-ray discs, are commonly used to backup personal computers. Some optical disc formats used for backups are of the WORM type, in which the data cannot be changed, while other optical storage systems allow for data backups without human contact, allowing for greater integrity.

Solid-state drives use integrated circuit assemblies to store data. These may include the use of flash memory, thumb drives, USB flash drives, memory sticks, and secure digital card devices.

Also known as cloud backups, remote backup services store data offsite, which can be valuable in the event of fires, floods, or similar disasters that could potentially destroy locally stored backups.

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