Bitcoin is the first and original distributed currency. The actual Bitcoin idea specifies how to securely and irreversably distribute or transmit "value" over the Internet.

Bitcoin is a massive distributed ledger of transactions, that is held by every Bitcoin user. To send value, I announce that I am transfering some Bitcoins to another user. The announcement validated and spread by the network, each user checking first, that I had the Bitcoins to send, then confirming the transmission.

The development effort behind Bitcoin is strong. The Bitcoin foundation has been established with the big players and smaller ones providing support for the currency to maintain its openness and independance.