Install Android ADB and Fastboot Tools

These two tools allow you to send terminal commands to your phone from your computer via USB. Both of these tools come with the Android SDK, however this is an extremely large download that, frankly, most users who are interested in ADB and fastboot don't need. They can be installed with relative ease at the same time, so its helpful to have both. Here is a very brief breakdown on what these tools do:

• Android Debug Bridge (ADB): This tool allows you to send a wide array of terminal commands—including but not limited to basic Linux shell commands, plus some specialty developer commands.
• Fastboot: When you need to modify your phone's firmware, fastboot is the tool you need. This allows you to send commands to the bootloader, which means you can flash/modify things like custom recoveries. 

If you're only interested in ADB, you can use this 15 second ADB Installer, however we're going to go ahead and kill two birds with one stone and get fastboot while we're at it (which is handy for unlocking those lovely Nexus devices). For that, we'll use the Minimal ADB and Fastboot tool. Download and run the .exe file in this XDA thread. This will automatically install ADB and fastboot. 

However, you can also modify your PATH variable to include the folder ADB and fastboot were installed to (by default, it's C:\Program Files (x86)\Minimal ADB and Fastboot\) , so you can run those commands from any command prompt no matter what folder you're in.