The device descriptor of a USB device represents the entire device. As a result a USB device can only have one device descriptor. It specifies some basic, yet important information about the device such as the supported USB version, maximum packet size, vendor and product IDs and the number of possible configurations the device can have.

The device descriptor is the first descriptor read by the host during enumeration. The purpose of the device descriptor is to let the host know what specification of USB the device complies with and how many possible configurations are available on the device.

A Descriptor contains information like type of the device, Vendor ID, Compliant USB Version, number of configurations device supports, number of endpoints, etc. The USB peripheral must respond with descriptors when requested by the host. During enumeration, the host uses control transfer to request the device for descriptors.

Each USB device can only have a single Device Descriptor. This descriptor contains information that applies globally to the device, such as serial number, vendor ID, product ID, etc. The device descriptor also has information about the device class. The host PC can use this information to help determine what driver to load for the device.

USB devices report their attributes using descriptors, which are data structures with a defined format. Each descriptor begins with a byte-wide field containing the total number of bytes in the descriptor followed by a byte-wide field identifying the descriptor type.