What is USB?
USB is a standard for ports (the physical holes on a computer) where devices can be plugged into via their cables. More specifically, it used be a single standard, but nowadays we have multiple USB standards: USB 1, USB 2, USB 3, Mini USB, Micro USB, and so on.
Before USB, many devices had their own ports, e.g. if a mouse and a keyboard used PS/2 ports and a computer didn't have two PS/2 ports, it couldn't have that mouse and a keyboard plugged into it. Printers and scanners also had their own ports. USB sought to standardize this. Today, you purchase a motherboard with 6 USB ports, for example, and you can just plug anything into it, with some exceptions.
Monitors don't use USB but a special port, such as be VGA, HDMI, or DVI, so it's possible to purchase a monitor that only supports one protocol such as HDMI and have a graphics card that only supports a different protocol such as DVI, and then you can't plug the monitor into the computer without an adapter. The reason monitors can't use USB, I assume, is because monitors receive a very large amount of data constantly with a very high frequency.
Some audio devices don't use USB but special audio plugs connected to an audio card. The reason for this is that the USB transmits digital signals (on-or-off voltage). The audio card converts digital signals from the CPU into analog signals (varying voltage). This varying voltage is converted into sound waves simply by emitting vibrations accordingly and amplifying them. If USB is used, this conversion would need to be done inside the headphones or speakers circuitry instead of inside the motherboard's audio card.
Backwards Compatibility
Although you can't plug an USB connector into a Mini USB or Micro USB port, you can plug an USB 1.0 connector in a USB 2.0 port or USB 3.0 port. They're designed to be backwards compatible. So if you buy a computer that only has one USB 3.0 port, you can use it as a normal USB 2.0 port.