the vast majority of defi apps rely heavily on oracles, middleware entities that connect smart contracts to resources outside of their native blockchains. Without oracles, blockchains are like computers without internet access... Often referred to as hybrid smart contracts ... [oracles enable] smart contracts that can react to real-world events a…

"a smart contract is a contract that enforces itself. ... a computer program that can be run on hardware which automatically executes those conditions." Good intro to what can - and can't - happen once you have bitcoins. - DAOs Are Not Scary, Part 1: Self-Enforcing Contracts And Factum Law – Bitcoin Magazine