This page was spun off of a User contributed FAQ due to space considerations on the FAQ page. In addition, before doing any of these procedures, the precautions outlined on the upgrade FAQ are recommended. But you should keep in mind the difference between each of these procedures below once precautions have been taken.
"Clean install" is not a Tiger (Mac OS X 10.4) installation option, nor an option to any version of Mac OS X. If you hear people say "Clean Install" about Mac OS X, please correct them as that is not the term Apple uses for Mac OS X installation.
Windows users are used to a clean install process which closely approximates the Mac OS X "Erase and Install." Mac users who have used Mac OS 9 and earlier, are used to a clean install process which closely approximates Mac OS X Archive and Install. As such it is recommended you only use the term Apple uses in its installer so that when other people discuss with you what installation to perform, your communication is clear. Otherwise one person may think you meant to erase the hard drive, while another thinks you only meant to disable the old operating system folder, and create a new one.
Clean install is what you do to Mac OS 8 and 9 ( 8.0, 8.1, 8.5, 8.6, 9.0 - 9.2.1) when you create a new System Folder, and rename the old one Previous System Folder and have to manually move Preferences, control panels, and extensions over to the new system folder from your old application installations.
In Mac OS X, you have five installation options, one of which is like a Clean Install but goes under a different name. There is:
1. Erase and install - erases the hard drive and installs a fresh copy of the operating system. Currently not recommended because of a bug which began in 10.3 that has caused some Firewire hard drives to become unreadable after installation of 10.3 or later. Unless of course you have two copies external of your internal hard drive of every critical file and know how to access them easily, erase and install will only leave you with the backup of your data, assuming the backup is accessible after the installation of the system. With 10.3 or later, there are no guarantees this will be the case with external Firewire hard drives. The problem has been narrowed down to Oxford 922 and 911 drives and Apple is working to solve this problem.
Note: When you erase and install, and if your machine is Mac OS 9 bootable, you need to make sure to add Mac OS 9 drivers when erasing the system to ensure that Mac OS 9 booting can happen after an erase and install.
For all intents and purposes, if instead of actually choosing erase and install, you use Disk Utility, or some other formatter to erase your hard drive before installation, you will have done the same thing as an erase and install. Granted, there is the zeroing with a Disk Utility which is not what an Apple erase and install does, but for purposes of simplicity and clarity, make sure when discussing what you do to install that you did this, rather than erase and install or clean install.
Another unintended side effect of an erase and install is that additional applications which were installed on the Mac initially have to be restored separately. With all other installation options on this page, the original applications which came with the Mac are preserved. To find out which original applications came with your Mac, I've constructed the Installed Applications on Mac OS X FAQ. Recovery partitions on 10.7 offers Disk Utility to do an erase, and then will allow you to reinstall the OS that shipped with your Mac, or the most current OS installed. Directions for recovery are available for Intel Macs, and M1 Macs.
Internet Recovery recovers the first system that shipped with the Mac, which is recommended only if you first backup your data and wipe the drive.
If you use just simple recovery, backup your data first, then run it from the main recovery screen without wiping your system, if all you want to do is make sure the current system isn't missing any components that are part of the operating system. Use Disk Utility to wipe the system only if you feel the system is unusable and you need to wipe it. When you use Disk Utility from recovery to wipe from recovery, be sure to wipe the top most partition after selecting View menu -> Show All Devices.
If wiping to restore Sierra (10.12.6) or earlier, be sure to reformat the disk utility Mac OS Extended Journaled. APFS won't work with it.
2. Restore system - restores the operating system and any applications that came with the Mac. Apple's Knowledgebase Article 61802, and 301486 describes several methods of restoring your system.
3. Archive and Install - first appeared under retail 10.2 CDs, and later to became available on 10.3 update CDs, in addition to 10.3 and 10.4 retail. It is unknown at this time if the 10.4 update CDs have this option. This gives you the option to create a new system folder, while renaming the old, and save user and network preferences. If you don't save user and network preferences, your installed applications that are non-Apple, and your Users folder get moved to the Previous System folder along with the previous operating system. This is the one most like the Mac OS 8 and 9 Clean Install option, but it is not called that by Apple. This is the currently recommended way to install a new operating system to make sure you preserve your data. Archive and Install may leave behind newer applications than will run with the operating system if you Archive and Install 10.2 over 10.3. It is recommended you move Apple programs replaced by Archive and Install out of the Applications folder before you Archive and Install 10.2 over 10.3. This option was terminated on the release of 10.6.
4. Upgrade Install - Available on upgrade disks of all versions, as well as retail disks. This will simply upgrade an existing X system to a new one, not concerning itself if any old system preferencees are incompatible with the new system. This is usually not the recommended option because of its lack of sensitivity to the possibility an old system may not have compatible parts. It is however, also the installation which happens if you select no installation option. As such, it is recommended you only do this if you have no non-Apple applications installed, and all your Apple applications are up to date.
When you install from the App Store an operating system that's new and your Mac supports, that's equivalent to an upgrade and install.
5. Install Mac OS X for the first time. When you install Mac OS X for the first time, it will happen when your machine never had Mac OS X installed before, or you erased the drive manually with a hard drive formatter and it detects there is no Mac OS X on the system to be installed. Naturally, if there was a system already installed on your computer that was pre-Mac OS X version of the Mac operating system, it won't touch that system if the hard drive is in good shape. Be sure of course to follow the migration tips on Migrating from Mac OS 9 to X FAQ before doing this.
To continue learning how to do an update of Mac OS X, please see the Upgrade FAQ.