Fastboot is a simple tool which makes it possible for the Android smartphone to connect with the computer. It is used to communicate with your phone only when it is in bootloader mode. Fastboot is a protocol designed to flash signed and unsigned partitions to Android phones directly in the flash memory of the phone. So if you are familiar with ADB think of it in the same way.
Fastboot is the safest way to install different firmwares on any Android device. Fastboot is designed to help flashing images such as bootloaders, kernels and recoveries on your Android phone.
Fastboot For Windows
You can download Fastboot to your computer from Google’s SDK. Download it here. Download the proper package, depending on your system 32 or 64 bit. You also need to install the drivers to allow the PC and phone to communicate. Search for your phone drivers and install or you can use PadaNet. Once you have downloaded the SDK package look for Fastboot. If you want to download only Fastboot and ADB, download this useful tool here.
Fastboot For Linux
To install Fastboot on Linux at first you must download the appropriate SDK package. Download it here. After SDK is installed, extract contents in any folder. You also need to have the latest java JDK installed. Download it here.
If you want to learn more about Fastboot, how to use it, unlock bootloader and flash custom ROMs and kernels visit the original thread here.