An extension when having a male to female converter or adapter at one end. Generally when picking up any extra cable you want to know what is used for like a printer that also see a male plug at each end but one is smaller in size. The same is seen for usb cables for use with cell phones, digital cameras, and other hand held devices to connect to pc.
I wouldn't recommend doing much charging of devices like cell phones by usb however especially when having a hub in use. It seems the phones will pull a bit more where the ac charging adapter can handle the extra draw seen at times.