Purpose and Features

• Before USB ports became a common standard for connecting peripherals, computers had a variety of input/output ports for specific purposes such as parallel ports for printers, serial ports for communication devices, PS/2 ports for keyboard and mice and game ports for joysticks. Computers needed to become more user-friendly to gain more widespread acceptance, and the USB port posed a solution for this problem because it supported many different types of devices. Rather than needing to remember which type of port goes with a particular device, a user can connect all of her peripherals to one type of port.
  
  

Types of Devices

• USB supports nearly any type of device that can be connected to a computer. Some common USB devices include hard drives and flash drives for storage, modems and wireless adapters for communication, printers and scanners, webcams and game controllers. Because virtually all computers have USB ports, hardware manufacturers tend to devote much of their attention toward making products for USB.
  
  

USB and the iMac

• As of 2011, Apple offers two versions of the iMac: one with a 21.5-inch display and one with a 27-inch display. Both computer models feature four USB 2.0 ports, along with other connectivity options such as FireWire 800 and Thunderbolt. The iMac does not include any USB 3.0 ports.
  
  

USB Limitations

• The primary limitation of USB is that it does not support a direct connection between two computers. FireWire and Thunderbolt both support this. If you attempt to connect two computers with a USB cable, however, you can damage both computers. Additionally, a specific limitation of the iMac's USB connectivity is that it includes no USB 3.0 ports. USB 2.0 can transfer data at a maximum rate of 480 megabits per second (Mbps), while the top speed of USB 3.0 is 5 gigabits per second (Gbps). Apple has chosen to implement the faster Thunderbolt connection -- which has a top speed of 10Gbps -- rather than including a USB 3.0 port on the iMac.