Apple stopped making their software available on CD/DVD ages ago. Applications including the entire OS is downloaded only through their App Store.
When you download and install the OS from their App store, it performs an upgrade to the existing operating system. Your applications and a large majority of your settings are preserved. But what if you want to perform a clean install? Wipe everything off and start anew? Well to do that, you’ll need to have the installer on another media. Such as another hard drive you can connect to, a CD/DVD, or flash memory (thumb drive, SD card, etc.).
To transfer the OS installer to a secondary storage media requires more than just copying the installer to the media. It has to be bootable. You will need to install the OS on the secondary media. The general method is to format the target destination (flash memory, hard drive, etc.) and run the installer on the target.
createinstallmedia is an easy way of installing the operating system to the secondary storage media. This was introduced in Mavericks. (Unfortunately I didn’t know about it until now because I skipped Mavericks.)
createinstallmedia is packaged in their OS installer. There is no need to download it separately.
createinstallmedia is a program that needs to be run in Terminal.
There are numerous tutorials available online (video and writeup). From what I’ve read and seen, the steps require downloading the OS installer, formatting the target media, copying the code and pasting into Terminal, and then executing the code. The tutorial steps are straight forward. If you don’t deviate from them, then you will succeed with a bootable system.
For my tutorial, I’m targeting the command line averse. The method I employ gives more leeway. For example, the majority of the tutorials online have you download the installer from the App Store and leave it alone (that is, don’t move it from the Applications folder). Well, say you’ve downloaded the OS already and have is stored in another folder or another volume (a different hard drive)? If you try to execute the command based on their tutorial, it will fail because the path is wrong. The method I use will ensure the path is filled out correctly. I’m using a USB flash/thumb drive as my target destination, but this should work on any other form of flash memory or external hard drive. The only caveat is the target destination must be large enough to accept the OS (min is about 6GB; 8GB is ideal).