Hi Lynman You are using a mixing desk into your line or mic imput. that is a good way of recording into your pc if you only need to lay down one track or chanel recorded at a time.
A special usb mic plugs into a usb port and is either passive or gets it's power from the usb with no mixer involved. You can also just plug a mic or any other imput straight into the mic or line imput and it works just as well. All these can only record one chanel at a time.
With an interface you can record several instruments and mics simultaneously and each one is laid down on a different track or chanel on your software. Makes recording and post editing a lot easier.