First things first—What does "Custom ROM" mean?
Most Android devices come with so-called NAND memories. A NAND memory is a particular type of flash memory. A flash memory is based on transistors, instead of rotating disks, like in old hard drives. This type of memory is completely electrically managed—it can be written and erased and can store data indefinitely (not volatile). Knowing this, we may think that everything is writable on Android. Well, not exactly!
The acronym ROM stands for Read-Only Memory. This type of memory is often used in embedded systems to safely store all those files that are part of the core system. In an effort to guarantee the highest system integrity possible, developers must be sure that the core system stays intact over device reboots and possible failures. That's why the core system is stored in a type of memory that can only be written once—Read-Only Memory, to be precise. With time, the Android hacking community took the acronym and transformed...
