In cryptocurrency, proof-of-stake is an alternative to proof-of-work, which avoids computational waste by requiring the prover to show ownership of money. The caveat is that the only types of money that work with a proof-of-stake system are cryptocurrencies, and thus the technology has only recently become possible.

FACT: There have been several proposals on how proof-of-stake can be implemented; the only one that is currently working in practice, however, is PPCoin (Peercoin), once again created by Sunny King.

This video from PPCoin describes how Peercoin is different from other coins like bitcoin, one of the most important ways being that it uses proof-of-stake rather than proof-of-work.