USB header connectors on a computer's motherboard provide a USB signal for the ports located on the front or side panel of desktop and laptop computer cases. A typical header will have nine pins, enough to provide a full USB four-wire connection to two different ports. The ninth pin connects to a shield wire that carries no data but protects the USB connection against electromagnetic interference.
Vcc Pin
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One of the four key connectors on a USB header is the Vcc pin. Also referred to as Vbus or V+, the pin carries a 5-volt signal that provides up to 500 milliamps of power for connected USB devices. While the "V" in Vcc refers to the voltage that it provides, the "cc" is not short for anything. The "cc" refers to a positive voltage connected to the collector terminal of a transistor. Double-letter codes for voltages start with bb and run through ss depending on the type of voltage and the place where it is to be connected.
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+D and -D Pins
These two pins carry upstream and downstream data connections. As a serial connection, USB transfers one bit at a time sequentially. Data speeds vary depending on the flavor of USB; for example, USB 2.0 has a 480 Mbps transfer rate, while USB 3.0 is capable of speeds as fast as 5GB per second.
Ground Pin
The ground pin provides a neutral "ground" connection. This pin provides a place for the current from the Vcc connection to flow back, completing the circuit. A ground pin is necessary to prevent electric shock in devices using large quantities of power. In low-voltage USB connections, it not only completes the circuit but also can reduce electromagnetic interference.
Purpose of Headers
Motherboards provide USB headers to allow cases to have front-mounted USB ports. Motherboards need to work in a wide range of cases, and headers provide a place for the case's ports to connect to the board. Offering front-panel connections also lets motherboard builders save money by putting fewer USB ports on the back panel.