Ripple (xrp) is the currency for Ripple network, and can be used throughout Ripple. A total of 100 billion xrp is issued, but is gradually decreasing as transactions increase. Ripple network is operated by Ripple Labs (former Opencoin).
Xrp is the only currency that can be used throughout the ripple network without restrictions. Other currencies such as cny and usd cannot be cashed out across all gateways. For example, CNY issued by gateway A can only be withdrawn at gateway A. If you wish to withdraw it from gateway B, you must first use Ripple’s pending order system to convert it to the CNY of the B gateway. Xrp has no such restrictions at all, it is universal in the ripple system.
Like Bitcoin, the Xrp is a digital currency based on math and cryptography. However Xrp is the main bridge currency and guarantees security features in the Ripple system, which Bitcoin fails to do. The security function is indispensable, and cooperate partners must hold at least some Xrp.