Today, most computers come with two or more USB ports and it's very unusual if your computer doesn't have at least one.
Right now there are four main types of USB connectors: Type A, Type B, Mini-A and Mini-B. All of these types are supported by USB 1x and USB 2x. Let's go over what these different connectors do in relation to what you do with your USB ports.
This type is known as the host connector, also referred to as the downstream connector. This is the one that you usually plug into the ports on your CPU or into a USB hub. It's usually rectangular in shape.
This connector is mainly used for the devices that can be plugged into a port. This one is also known as the upstream connector and is the one that attaches directly to the end of a device (whether that be a digital camera, a printer, a mouse or keyboard, etc). A Type B connector is usually box shaped.
This type of connector was designed to connect with mobile devices. This one can be used without a computer and it can be used with most mobiles, big and small.
This connector was designed to replace the original connector for USB 2.0 because they were too big for certain devices such as a PDA or a cell phone.