TWRP is a custom recovery software for Android. This guide illustrates how to safely install TWRP to an Android device without a root.
There is now a way to create a flashable ZIP of your favorite Android apps, and you can flash it to install all apps on your device in one go. Here is how you can do it.
There are hundreds of custom ROMs available on the Internet, and the most efficient way to get them installed on your device is to use a custom recovery.