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Ripple credits, aka XRP or ripples, are the units used as a transaction fee to protect the Ripple payment system from malicious attacks.
XRP is the proposed ISO 4217 currency code designated for Ripple.
XRP as currently implemented has six digits of precision. The smallest unit is one millionth of an XRP ( 0.000001 ) and is call a "drop". That is: 1 XRP = 1,000,000 drops.
XRP spent as transaction fees are destroyed. The default transaction fee is currently 10 drops. There should be enough XRP to last for thousands of years despite this destruction. If the value of XRP changes, the transaction fee can be adjusted by the consensus of the network.
XRP are the only currency in the Ripple network that has no counter-party risk and can be sent to any account without a trust relationship.
When the Ripple network was created, 100 billion XRP was created. The founders gave 80 billion XRP to the OpenCoin Inc. OpenCoin Inc. will develop the Ripple software, promote the Ripple payment system, give away XRP, and sell XRP.
https://ripple.com/wiki/Ripple_credits