Sounds similar to my four quid Asda one, which has a usb socket in one end and a micro-usb socket in the other end, and it comes with a short, standard usb to micro usb cable.
You charge it with the usb plug of the cable in an adapter in a mains socket, or in a mains socket that has a usb socket in it, and the micro-usb end of the cable plugged into the powerbank.
When you use the powerbank to charge the phone, you use the same cable plugged into the usb end of the powerbank and the micro-usb end of the cable plugged into the phone.