The Google USB Driver is required for Windows only in order to perform adb debugging with any of the Google Nexus devices. The one exception is the Galaxy Nexus: the driver for Galaxy Nexus is distributed by Samsung (listed as model SCH-I515).
Windows drivers for all other devices are provided by the respective hardware manufacturer, as listed in the OEM USB Drivers document.
Note: If you're developing on Mac OS X or Linux, then you do not need to install a USB driver. Instead see Using Hardware Devices.
You can download the Google USB Driver for Windows in one of two ways:
Select Google USB Driver and click OK.
Using the SDK Manager helps you keep the driver up to date by notifying you when your current driver is out of date.
For installation information, read Installing a USB Driver.