An incremental backup is a backup of all the files that have changed since the last backup. Typically, you perform a full backup, then a series of incremental backups.
You can perform full and incremental backups using tools like BackupExec or NTBACKUP.EXE. All commercial backups can do incrementals.
Incremental backups take far less space than full backups, and also take a lot less time to perform. In some situations, it's feasible to run backups during the workday.
Restoration of files from an incremental backup are performed by restoring the latest version of a file. This is done automatically.
Generally, previous versions can also be restored, so incremental backups also serve as a way to archive changes to the file system.
In many cases, incremental backups are better for archives than full backups for archives. For one, files that are created, and then later deleted, in the interval between full backups, are not stored in the archival backup. The problem is, basically, size - because a full backup is the same size as all the files. Keeping incrementals as well requires the full size, plus space for all the incrementals.
Incremental backups after periodic full backups are the preferred way to perform backups.