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.
Figure 1. The SDK Manager with the Google USB Driver selected
<sdk>\extras\google\usb_driver\
directory.Using the SDK Manager helps you keep the driver up to date by notifying you when your current driver is out of date.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.