What is a Custom Rom

 The Term ROM (Read Only Memory) refers basically to firmware which contains OS (operating system) and Kernel. Kernel is heart of operating system which makes everything operate smoothly. All Android devices come with a Stock ROM which is nothing but the version of Phone’s OS that comes pre-loaded with your phone when you buy it.A custom rom is a customized version of stock rom,a standalone version of OS including the kernel which makes everything operate/run. The only difference between a Custom ROM and Stock ROM is that Stock ROM comes from manufacturers whereas Custom ROMs for Android is a modified standalone version given by android developers. Since Android is an open source OS, developers provide custom ROMs for Android after modifying, optimizing, removing bloatwares from its Stock ROM.