It’s very handy to have a bootable version of OS X on a thumb drive.
I have written how to do this using a program called DiskMaker X,  and I have written about how to do it from the command line here,  but after doing this a few times I find the most reliable method is the one outlined below.  It uses Apple’s built-in disk utility.This works with any version of OS X. Mountain Lion, Yosemite, El Capitan etc. You’ll need an 8G USB thumbdrive.
Since OS-X Mavericks (10.9) Â Apple have been releasing their OS X upgrades for free. This means if you have any version of OS X newer than 10.9 then you can upgrade it to the latest version for free. Â There are three ways you can upgrade.
Method 1 is Apple’s suggestion method, but if you have more than one computer it requires you to download the OS X update more than once.  Method 3 gives you a thumb drive that you can use multiple times to install OSX,  and you can even boot off it  in an emergency if your hard drive crashes.  It is very handy to have a bootable version of  the latest OSX on a thumb drive lying around  So I suggest option three
This article tells you how to make a USB thumb drive installer so you can upgrade any computer to the latest OS X.
It also tells you how to work out what is the newest version of OSX that you can run on your computer.Â
Here’s how to make a bootable installer of El Capitan on a USB Thumbdrive.  It is the quickest way but it uses the terminal so it is for advanced users. If you want a way that is slightly longer but easier for beginners download and run Diskmaker which does the steps below for you automatically!
This is an advanced topic for people wanting to host a wordpress site on their own mac.
If you have a Lion installer on a USB thumbdrive, you can use it to upgrade your Mac from Snow Leopard to Lion, but if you have an older version of OS X you can’t upgrade. You need to first upgrade to Snow Leopard OR you can erase everything and do a completely fresh install of Lion.  After installing Lion on my iMac, I wanted to delete everything on my laptop and do a fresh install of Lion. But the installer will not install over an older version of OSX. I had to delete the old version of OSX, then install Lion from the USB thumbdrive. Here’s how to do a fresh install of Lion.
The new OSX Lion comes as an upgrade from the Apple App Store – not on a DVD. The good news is that once you’ve paid for it once,  Apple allow you to install it on all the computers you own for free! After you purchase it from the App Store you  just have to go into the App Store on your other computers and Lion will be there – all paid for and ready to download again!
The Apple page on this is here.
Of course, you need to upgrade all your existing computers to Snow-Leopard first.
The main hitch is that it needs to download lots of files first.
You can make a USB installer and copy it across manually. Here is a way to do this using the built-in Disk Utility in OS X.
In Snow Leopard if you try to install a program while another is installing you get an error message saying “Waiting for other installations to complete.” But what do you do if the message stays there after the program has finished installing?