Casper is a security-deposit based economic consensus protocol.
Security-deposit based security and authentication
Casper is a security-deposit based economic consensus protocol. This means that nodes, so called “bonded validators”, have to place a security deposit (an action we call “bonding”) in order to serve the consensus by producing blocks. The protocol’s direct control of these security deposits is the primary way in which Casper affects the incentives of validators. Specifically, if a validator produces anything that Casper considers “invalid”, their deposit are forfeited along with the privilege of participating in the consensus process. The use of security deposits addresses the “nothing at stake” problem; that behaving badly is not expensive. There is something at stake, and bonded validators who misbehave in an objectively verifiable manner will lose it.
Or simply
Casper will disincentivize any attack vector against the Ethereum network, making it expensive for an adversary to bribe a high number of nodes to undermine the entire protocol.
More Details
