OS X Mavericks is usually installed as an upgrade over an existing version of OS X (Snow Leopard or later). But the Mavericks installer you purchase and download from the Mac App Store can do much more. It can perform a clean install on a freshly erased startup drive, or a new install on a non-startup drive. With a bit of fiddling, you can also use it to create a bootable installer on a USB flash drive.
All of these installation methods make use of the same Mavericks installer. All you need to use these alternate install methods is a bit of time and a handy guide, which we happen to have right here.
The minimum requirements for OS X Mavericks haven't changed much from the minimum requirements for OS X Mountain Lion. And that makes sense since Mavericks is really just an upgrade to Mountain Lion, and not a wholesale rewrite of the OS.
Nevertheless, there are a few changes to the minimum requirements, so be sure to check them out before proceeding with the installation.
Having a bootable copy of the OS X Mavericks installer isn't a requirement for the basic installation of Mavericks on a Mac. But it's handy to have for the more complex installation options. It also makes a terrific troubleshooting utility that you can take with you to work on the Mac of a friend, colleague, or family member who is having problems.
As a troubleshooting utility, you can use the USB flash drive to boot a Mac that's having problems, use Terminal and Disk Utility to correct the problems, and then reinstall Mavericks, if necessary.
The upgrade install of OS X Mavericks is bound to be the most often used installation method. It's the default method the installer uses, and will work on any Mac that has OS X Snow Leopard or later installed.
The upgrade install method has some very practical benefits; it will install over existing versions of OS X without removing any of your personal user data. Because it retains all of your data, the upgrade process is a bit quicker than the other options, and you don't have to go through the setup process of creating administrator accounts or Apple and iCloud IDs (assuming you already have these IDs).
The upgrade install is recommended for most users because it will let you get back to working with your Mac faster than any other installation method.
Clean install, fresh install, it's all the same thing. The idea is that you're installing OS X Mavericks on a startup drive and wiping out all of the data that is currently on the drive. This includes any existing OS and user data; in short, anything and everything.
The reason for performing a clean install is to get rid of any issues you may have with your Mac that are caused by an accumulation of system updates, driver updates, app installs, and app removals. Over the years, a Mac (or any computer) can accumulate a lot of junk.
Performing a clean install lets you start over, just like the first day you started up your shiny new Mac. With a clean install, most issues you may be experiencing with your Mac, such as freezes, random shutdowns or restarts, apps not starting or failing to quit, or your Mac shutting down slowly or failing to sleep, should be corrected. But remember, the cost of a clean install is the loss of your user data and apps. You'll have to reinstall your apps and any user data you need.