Sometimes referred to simply as a backup, an archive contains copies of your files as they appeared at multiple points in time. If you want to see the version of a file that existed on your computer 2 weeks ago, an archive can deliver thatalong with today's version and the version that existed a month ago.
An archive starts with a complete copy of all the files in one or more folders. The next time the backup runs, your backup software could make another complete copy, but because most of the files probably have not changed in the meantime, that would use up a great deal of spacenot to mention taking a long time. So backup programs typically perform an incremental archive. This means that on subsequent runs, the software scans the files in the folders you've designated and copies only those files that are new (or newly modified) since the last backup. To be truly useful, archives should also be additive, meaning the backup program adds the new or changed files to the archive without overwriting the files already there. That way, you can retrieve many different versions of a given file, and if you delete it on your hard disk, you can still find it in your archive. Thus, what I refer to as an archive is technically an additive incremental archive.
Some backup programs use the term archive to describe files that have been copied to removable media of some kind for long-term storage and then deleted from the source volume.
Archives sometimes make use of a snapshota list of all the files in the designated folders at the time a backup runs. Even though a certain file may not be copied (because it hasn't changed since the last backup), it will appear in the snapshot list. You can easily see what the entire contents of a folder looked like at various arbitrary points in the past, and restore it to any previous state in a single operation.
After the initial full backup, archives usually take comparatively little time to run, making it easy to back up your data once (or even several times) each day. This ensures that your most recent backup is never more than a day old. Because they also offer tremendous insurance against accidental deletion (or change) and file damage, archives are an essential part of a good backup strategy. But archives alone are not an adequate solution. I say this for two main reasons:
Archives protect you against inadvertent changes over time, but only a duplicate can get you up and running again quickly after a major problem. In other words, the best backup strategy includes both duplicates and archives.
That said, you can set up both duplicates and archives in many different ways, depending on the hardware and software you have, the types and sizes of files you typically work with, and other variables. I make some general suggestions ahead under Joe's Recommended Strategy (page 104), and I provide more detailed instructions in Chapter 12.