What is Fastboot?

Fastboot is a command line tool used to directly flash the filesystem in Android devices from a host via USB. It allows flashing of unsigned partition images. It is disabled in most production devices since USB support has been disabled in the bootloader. In order to use Fastboot, you must have an unlocked bootloader or an Engineering SPL

Pre-requisites

- You must have ADB drivers installed for your device. Some devices have fastboot specific drivers too, so Google that for your device.

- Fastboot package, available HERE. 

Typical Partition Layout

Examples

Manual Nandroid restore

Make a Nandroid backup, and copy the folder off your SD card to your desktop.

cd ~/Desktop/path/to/backup/
fastboot flash userdata data.img
fastboot flash system system.img
fastboot flash boot boot.img
fastboot reboot

Flash Custom Recovery.img

fastboot flash recovery /path/to/.img
fastboot reboot

Flash Custom boot.img

You should make a Nandroid backup first, in case you need to recover your boot.img

fastboot flash boot /path/to/.img
fastboot reboot

Development

If you are developing a kernel, you may find it helpful to delete your boot.img and recovery.img to force booting into fastboot:

fastboot erase boot
fastboot erase recovery

You can then manually boot from your custom kernel and a ramdisk:

fastboot boot

Once you have a working kernel and ramdisk, you can automagically combine them within fastboot:

fastboot flash:raw boot

NOTE - THANKS TO THE CYANOGENMOD WIKI FOR THE BASE FOR THIS GUIDE