Swarms of small bitcoin transactions directed into semi-public addresses that require proof of work to claim.
Less literally, they are a form of e-currency (synthetic commodity with cryptographically enforced scarcity) that is supply elastic, but can be easily traded for or converted to bitcoins with no centralized exchange.
Throughout this document, I will refer to "proto" as the money-like substance and Proto as the protocol or client. Note: This will not alter the existing bitcoin in any way, it is just a new client that leverages its cryptographic capabilities in new ways. Proto does not yet exist.