Square Up is a service for taking payments by credit card that works with a special "Square" device that plugs into an iOS device's headphone jack.
Other hardware options are available also.
Square Cash is a different service for exchanging money using debit or credit card payments. Creates a simple-looking cash.me/$username URL to share for people to pay you. An amount can be pre-selected by adding it to the URL, e.g. https://cash.me/$username/10.