Ripple (XRP) is the native currency of the Ripple network that only exists within the Ripple system.
Ripple does not rely on proof-of-work concept. Instead, consensus protocol is used in order to validate account balances and transactions.
One of the specific functions of XRP is as a bridge currency, which can be necessary if no direct exchange is available between two currencies at a specific time, for example when transacting between two rarely traded currency pairs.
All of Ripple 100 billion coins were already „pre-mined“; any transaction requires a commission fee granting that total coins count can only decrease with time.