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  Universal Serial Bus (USB) was developed in the 1990s in an effort to simplify the connections between computers and peripheral devices. It has become widely popular due to its compatibility with many platforms and operating systems, its low cost of implementation, and its ease of use. Most computers that are built today come with several USB ports, and USB is the interface of choice for most home and office peripherals including printers, cameras, modems, and portable storage devices. USB standards are developed and maintained by an industry body called the USB Implementers Forum (USB-IF). In its original specification, USB defined only two connector types: A and B. Revisions to the specification and demands on manufacturers have expanded the breadth of connectors used for USB devices, but the majority of USB products still use these A and B connector interfaces.

USB A-Type


USB A-Type Male


USB A-Type Female
  Found on host controllers and hubs, the A-style connector is a flat, rectangular interface. This interface holds the connection in place, and makes it very easy for users to plug and unplug. Instead of pins, the connector uses flat contacts which can withstand continuous attachment and removal very well. The A-socket connector provides a 'downstream' connection that is intended for use solely on host controllers and hubs. It was not intended for use as an 'upstream' connector on a peripheral device. This is critical because a host controller or hub is designed to provide 5VDC power on one of the USB pins. That is also why A-A cables are not common. Note: An A-A cable, if used to connect two downstream host controller or hub ports together, may short the hosts power supplies. This could cause damage to equipment and may even present a fire hazard.

USB B-Type
USB B-Type Male
  
  The B-style connector is designed for use on USB peripheral devices. The B-style interface is squarish in shape, and has slightly beveled corners on the top ends of the connector. Like the A connector, it uses the friction of the connector body to stay in place. The B-socket is an 'upstream' connector that is only used on peripheral devices. Because of this, the majority of USB applications require an A-B cable.

USB mini-B (5-pin) USB mini-B (5-pin)
USB mini-B (5-pin)
  
  One drawback to the B-style connector is its size, which measures almost 1/2-inch on each side. This makes a B-style interface unsuitable for many compact personal electronic devices, such as PDAs, digital cameras, and cellphones. As a result, many device manufacturers began using 'miniaturized' B-style connectors. The most popular style of mini-B connector, and the only one recognized by the USB-IF, is the 5-pin mini-B. It is quite small, about two-thirds the width of an A-style connector. The 5-pin mini-B is also specified for use in a new standard called USB On-The-Go which allows peripheral devices to communicate with the presence of a host controller.

USB mini-B 4-pin Olympus USB mini-B (4-pin)
USB mini-B (4-pin)
  
  This is an unofficial connector found on many digital cameras, especially certain Kodak models. It resembles the shape of a standard B-style connector, with beveled corners; however it is much smaller in size.

USB mini-B 4-Pin Fuji     
  Another 4-Pin connector also widely used on digital cameras, especially certain models manufactured by Fuji. It more closely resembles an A-style connector with its flat, rectangular shape.
Lots more Information on USB from the Folks at Wikipedia