Like Bitcoin:
Ripple is an open source, 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 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

https://ripple.com/wiki/Introduction_to_Ripple_for_Bitcoiners