At first glance a USB keyboard isn't really something special; it's exactly like any other keyword—except that it connects to your computer via a USB cable instead of the thick plugs that older computers use. In fact, there might be some disadvantage in one since many devices use USB these days, and so you might run out of unused slots. But a big advantage is that laptops, for instance, don't have those thick plugs, and so if you want to use a separate keyboard with your laptop you'll need a USB keyboard. There are also mini keyboards, which are exactly what you would think: tiny keyboards.
Once you look around here on eBay, you'll realise that the options are virtually limitless, and good value too. If you need a new keyboard, there really is no reason not to just order one right here, right now.