Proof-of-stake is a method of securing a cryptocurrency network through requesting users to show ownership of a certain amount of currency. It is different from proof-of-work systems that run hashing algorithms to validate electronic transactions. It is most commonly used as a supplement to proof-of-work in Peercoin and a few other electronic currencies.
Usage in Peercoin
Peercoin's proof-of-stake system is based around the concept of "coin age," a measure of the product of the currency amount held times the amount of time it has been held for. When generating a proof-of-stake block, the user sends some money to themselves, consuming their coin age in exchange for a preset reward. This minting transaction becomes more likely to succeed over time until a valid block is found, generating a new block on the blockchain and a payout for the proving user. This process secures the network and gradually produces new coins over time without consuming significant computational power.
Both proof-of-work and proof-of-stake blocks are used in Peercoin, although the main blockchain is determined by the highest total consumed coin age (from proof-of-stake generation) instead of the total combined difficulty of the chain (determined by proof-of-work blocks, as in Bitcoin). Peercoin's developer claims that this makes a malicious attack on the network more difficult.