Android Partitions

What is mean by partition?

Partition is a division of a disk created to give separate location/space to each directory/system software in operating system, operating system mainly contains boot, system, vendor, recovery, data, etc partitions. The File system used in android partitions is ext4/ext3. 

Vendor -

Vendor partition contains the applications,and other data provided by the companies, vendor partition can contain customized data.

command - fastboot flash vendor <vendor image file name>

System -

System partitions contains all the data which runs the Operating System, the system partition contains binary files, applications, bootanimation, build properties and all the data which run the OS. Custom System partitions can be flashed with fastboot if bootloader is unlocked.

command - fastboot flash system <system image file name>

Recovery -

The recovery partition contains the recovery which is a system software, if device gets bricked or have any problem to start it can recover using recovery software. The recovery partition can flashed with a custom recovery using fastboot. Stock recovery cannot flash the custom firmware because it is program to flash only stock firmware/OTA which contains oem's signature. The custom recovery softwares can flash any custom firmware.

command - fastboot flash recovery <recovery image file name> 

Boot -

The boot partition contains Bootloader, which is also a system software Bootloader verifies the system before booting it checks if system is running in stock firmware if not it blocks to device's boot process. Custom firmware/ custom recovery cannot be flashed while bootloader is locked. To use the custom partitions in device bootloader should be unlocked. Unlocked bootloader doesn't verify the Firmware It allows any firmware to boot. 

command - fastboot flash boot <boot image file name>