USB cables are used to connect something to your PC. Printers, scanners, hard drives, smart phones, and hundreds of other devices can connect to your computer with a USB cable. There are different kinds of USB cables with the big difference being the connectors on the end. USB cables can be up to 15' long without special repeaters.

 

This is the most common type of  USB cable. If you went to the store and asked for a USB cable, this is probably what they'd sell you. The Type A end connects to your computer. The Type B end connects to your device. This type of cable is usually used for larger devices such as printers, scanners, or full sized USB hard drives.
   
This is a type of USB cable that's becoming more common. The end on the right is a standard Type A connector that connects to your computer. The end on the left is known as a Mini-B connector. The Mini-B connector is becoming increasingly common and is typically used on smaller devices such as digital cameras, small hard drives, cell phones, and even Bluetooth headsets. As more small devices standardize on the Mini-B connector not only for connecting to a PC but for charging their batteries from a wall charger, travelers can pack one USB cable and one charger that works across many devices.
   
This a Firewire (IEEE 1394) cable. The ends of the cable are identical and contain 6 pins, 4 for data and 2 for supplying power to the device. The most common devices with Firewire connections are external hard drives and digital video cameras. Some Firewire devices, usually laptops, use the smaller 4 pin connection that lacks the two power wires. Since most Firewire devices have their own batteries or plug into the wall for power, the two power wires on the cable aren't used often so its easy to connect 4 and 6 pin devices by using the adapter pictured.
   
This is a parallel printer cable, more properly called an IEEE 1284 cable. The end on the left connects to the back of your computer and the end on the right connects to your printer. Since fewer and fewer computers and printers come with parallel cables, this cable is fading into history.
   
This is a Category 5 (usually abbreviated as Cat 5) cable, most commonly used to connect a computer to an Ethernet network or high speed Internet such as cable or DSL. The ends of the cable are identical and contain 8 wires in a plug similar to a phone plug but wider.
   
IDE hard drives and optical drives inside a desktop computer get their electrical power from the power supply by a plug known as a 4 pin molex plug. SATA hard drives use a different connector consisting of 15 pins. The cable shown here allows a SATA hard drive to get electricity from a computer power supply that only has 4 pin molex plugs.
   
At left is a standard IDE ribbon cable. IDE cables consist or either 40 or 80 tiny wires all running side by side. This cable has three connectors on it. The blue end connects to the motherboard while the others connect to two IDE devices such as hard drives or DVD drives.

The cable in the middle connects an internal floppy disk drive to a motherboard. It consists of 34 wires all running side by side.

The cable at right is a SATA data cable. It is used to connect a SATA device (hard drive or optical drive) to a computer's motherboard so the two can communicate. Unlike IDE, SATA cables can only connect one device to the motherboard.