# Blockchains are a decentralized computation layer A [[smart contract]] is a program running on the Blockchain. "Validators" and "nodes" are servers that are monetarily incentivized to run the program in the smart contract. Think of it as a marketplace for program execution that all run a common virtual machine. Right now, it has some serious problems: it's slow, expensive, and costly to the environment. It has [[the right things wrong with it]]. It's also fault-tolerant, guaranteed to be execute identically,