Square is a new device that will allow you to make payments on your credit card from your mobile phone and accept payments via credit card from others on your mobile phone.
However, at this point there is some confusion as to who exactly owns the patent on the product, so your chances of using it at a place near you or anytime in the near future are not so good.
But if you can understand this techie gobbledegook (no offense but just admiration to the ones who can, of course!) you can build your own!