Ripple is the base currency of the Ripple network, which can be circulated throughout the ripple network. It has a total supply of 100 billion, and is gradually decreasing as the number of transactions increases. Ripple's operating company is Ripple Labs (formerly OpenCoin).
Ripple currency is the only common currency in the ripple system. It is different from other currencies in the system. For example, CNY and USD cannot be cashed out across gateways. In other words, the CNY issued by the A gateway can only be cashed out at the A gateway, not the B gateway. Otherwise, you have to convert it into CNY of B gateway via pending-order of ripple system. However, Ripple has no such restrictions at all. It is universal in the ripple system.
Ripple (XRP), same as Bitcoin, is a digital currency based on math and cryptography. But what different from the no-real-use Bitcoin is that XRP plays the role of connection and boasts security guarantee function in the Ripple system. Security-guarantee is indispensable, which requires that the gateway participating in this protocol must hold a small amount of XRP.