A custom spin is a version of the Linux operating system that has each package selected or deselected by the creator of the custom spin. This can be quite useful if you want to install any build of any flavor of Linux, exactly the same way on numerous machines. If you want two or more systems to have the exact same kernel, OS, server software, development environment and applications then a custom spin is a way to save significant time and effort. Imagine a USB drive that has everything you need. You can install EXACTLY the programs you want and need, without a single download. Then envision that you can actually boot from that USB drive. This is what a LiveUSB is all about. I can carry my Operating system and software on my key chain. If one of my clients has an issue with their computer, I can boot to my USB drive and test their computer’s hardware, verify their Internet connection, and even troubleshoot Windows disk errors without ever adding a single file or setting to their system. I can make an exact duplicate of any Fedora system to the USB drive. I can then install that exact same OS with everything on any computer anywhere. Or I can just boot to that exact environment and see if I really like the way it works before I install it on any system.