Without the dot, "cardio" is Rule #1 in Zombieland. With the dot, card.io is PayPal's latest acquisition: the startup formerly known as Lumber Labs, whose technology allows developers to integrate credit card payments into any iOS or Android app with no additional hardware.
The idea is pretty simple: use your smartphone's built-in camera to snap a picture of your credit card, and the details are automatically read and optionally processed. Transportation services like SideCar and Uber already use card.io to take on-the-spot payments, and many other developers--ranging from Lemon's "digital wallet" to BarTab's "buy a friend a drink" app to 1-800 CONTACTS's contact lens purchasing solution--are now using card.io's API.
PayPal and card.io both promise that "[t]he current card.io technology will remain available to developers for use in their own applications," though it seems inevitable that PayPal will incorporate card.io's credit card scanning into its own mobile apps. New developers can still sign up for card.io's Android or iOS SDKs, and at least in the near term, allowing others to continue using card.io's services doesn't hurt PayPal.
Things could get more interesting if and when PayPal integrates card.io's API with its existing, global payment systems. PayPal's own API still depends on manual entry of credit card information, which can be tedious on a tiny smartphone screen. And though card.io's image scanning technology works globally--all you need is a decent smartphone camera and an ISO-standard credit card--its payment processing service only works in the United States. When combined with PayPal's ability to send payments between multiple countries and currencies, card.io could suddenly become extremely useful to many more people.