Universal Serial  Bus (USB) is a serial bus standard to interface devices to a host computer, designed to allow many peripherals to be connected using a single standardized interface socket and to improve the Plug and playcapabilities by allowing hot swapping, providing power to low-consumption devices without the need for an external power supply and allowing many devices to be used without requiring manufacturer specific, individual device drivers to be installed.

USB can connect computer peripherals such as mice, keyboards,  joysticks, scanners, digital cameras, printers, personal media players, flash drives, and external hard drives.

The Universal Serial Bus has the following features:

1-The computer acts as the host.Up to 127 devices can connect to the host, either directly or by way of USB hubs.

2-Individual USB cables can run as long as 5 meters; with hubs, devices can be up to 30 meters (six cables’ worth) away from the host.

3-With USB 2.,the bus has a maximum data rate of 480 megabits per second.A USB cable has two wires for power (+5 volts and ground) and a twisted pair of wires to carry the data.

4-On the power wires, the computer can supply up to 500 milliamps of power at 5 volts.Low-power devices (such as mice) can draw their power directly from the bus.

5-High-power devices (such as printers) have their own power supplies and draw minimal power from the bus. Hubs can havetheir own power supplies to provide power to devices connected to the hub.

6-USB devices are hot-swappable, meaning you can plug them into the bus and unplug them any time.

Many USB devices can be put to sleep by the host computer when the computer enters a power-saving mod .

The Serial ATA is a storage-interface for connecting storage controllers (most commonly integrated into laptop computers and desktop motherboards) to mass storage devices (such as hard disk drives and optical drives).

SATA offers several compelling advantages over the older parallel ATA interface: reduced cable-bulk and cost (8 pins vs 80 pins), faster and more efficient data transfer, and the ability to remove or add devices while operating (hot swapping).

Serial ATA has distinct key advantages over its predecessor. Cables are very thin with small 7-pin connectors. They can be up to 3 feet (1 meter) in length, and are easily routed to stay out of the way allowing maximum airflow inside the case. SATA also has a far lower power requirement of just 250 mV compared to PATA’s 5-volt requirement, and with chip core voltages declining, this speaks well of SATA’s future. Serial ATA does away with Master/Slave configurations and drive jumpers.

The most obvious change with Serial ATA is the difference in drive cables. Other differences include ( ex : ATA ):

• Higher throughput
• Revised power connector
• Longer data cables
• Support for external drives (eSata)