DriveDroid is an app by XDA member FrozenCow, which lets you boot an OS (Operating System) from your phone. The app lets your phone act as a live USB, allowing you to run a complete OS from your phone.
DriveDroid can download the OS of your choice for you, saving you a lot of work trying to do it yourself. The app needs root access, and your phone should support UMS (USB mass storage mode). Most devices have this option, but not enabled by default. If this is the case, DriveDroid will prompt you to enable it. DrivDroid is compatible with devices running Android 2.2 or higher, so basically everyone is able to use it.