What is Ripple and how does it compare to Bitcoin? What is Ripple, and how does the system compare to Bitcoin?
This is answered in the official wiki:


  Like Bitcoin:
  
  
  ripple is a distributed peer-to-peer payment network
  ripple transactions are irreversible, sent over the Internet, and counterfeit proof
  ripple uses the same underlying cryptography as Bitcoin
  ripple has multi-signature support
  ripple has low to no transaction fees
  ripple servers can be run by anyone
  
  
  Unlike Bitcoin:
  
  
  ripple can send any currency
  ripple can automatically exchange currencies
  ripple transactions are fully confirmed in seconds
  ripple allows nicknames and gravatar icons for accounts
  ripple's reference client is a Firefox and Chrome add on
  ripple has no block chain download, clients are ready in seconds
  ripple has no mining or direct monetary reward for running a ripple server
  ripple has no currency risk as people can hold whatever fiat they want
  ripple solves the double spending problem with consensus instead of proof-of-work