Smart contract

A smart contract is an irreversible self-executing contract between two or more parties. Essentially, it is a computer code that underlines the conditions and terms of agreement of the contract and is publicly available on the blockchain network. Smart contracts allow different parties who often don't trust each other to transact without the need for a central authority, legal system or any other type of intermediary. 

DApps 

Decentralised applications (DApps) are digital applications or programs that run on a blockchain network and are made possible by “smart-contracts”. DApps leave room for innovation and make it possible to rethink everything that today still requires centralised authority.