What are smart contracts?
Smart contracts are automated digital agreements that are programmed. They are self-executing, incorruptible, and unalterable. They do not require any actions or the presence of third parties.
Two worlds of smart contracts
We can break smart contracts and financial transactions down into two worlds:
In one world, you want to send some notion of value between one actor (or group of actors) to another actor (or a group of actors). There has to be a representation of that value, the terms and conditions behind it, and an event to trigger it. This is what we call a financial contract and best implemented with a domain-specific language. This world has nothing to do with replacing a big company or this common notion that we might have with dapps.
In the other world, you want to write programs, maybe even replace a large company, perhaps solve something smaller. Those applications consist of a triangle:
The client - this is what runs on your computer.
The server - this is what runs on someone's server (or multiple servers).
The smart contract - the piece of the code that actually runs decentralized.
Marlowe : https://developers.cardano.org/docs/smart-contracts/marlowe/- a domain-specific language, it covers the world of financial contracts.
Plutus : https://developers.cardano.org/docs/smart-contracts/plutus/- a platform to write full applications that interact with the Cardano blockchain.
This content is from (https://developers.cardano.org/docs/smart-contracts/) and is included here so it can be translated.