USB cables have two ends, and each end can be different. On one end of the cable is the A connector. The other end, most likely, has the B connector.
The A connector is rectangular. It's what plugs into the computer. This connector is often called the upstream end.
The B connector has a D shape to it. This type of hole and cable connector is fitted on USB devices. It's called the downstream end.
For example, to connect a USB printer, you plug the A end of the cable into your laptop and the B end into the printer. Ta-da!
Here are some things to remember when buying a USB cable:
When you get a USB cable, you want a USB A-to-B cable.
If you want a USB extension cable, you want a USB A-to-A cable, or one that’s labeled as an extension.
You can get other cable ends as well, although they’re specific to the devices they plug into. For example, some MP3 players use a special USB cable with an A connector on one end and a specific dongle on the other end that plugs only into the MP3 player.