Rooting in android means to gain super user access(Administrator access) for which root access is requires as the
su binary should be installed to
This is usually done via a
Recovery.Every android phone have a in-built recovery which can generally only flash OTA updates and ROM's provided by the device’s manufacturer. So a
custom recovery like CWM,TWRP etc is installed.A custom recovery has more features and allows you to flash any suppoerted file.
Each android device has a bootloader which is something like BIOS in PC.And many phones comes with locked bootloader which prevents users from installing any other ROM. Hence unlocking bootloader is a common term related to rooted devices.
ROM stands for Read Only Memory. A custom ROM replaces your device's Android operating system normally stored in read-only memory with a Different OS. Installation of Custom Rom is the process of flashing a Custom ROM usually using a Custom Recovery/ADB