In the easiest form, it is an agreement between couple of multiple parties online that can be automatically executed on a transparent blockchain using cryptocurrency.
In the context of blockchains and cryptocurrencies, smart contracts are:
– pre-written logic (computer code),
– stored and replicated on a distributed storage platform (eg a blockchain),
– executed/run by a network of computers (usually the same ones running the blockchain),
– and can result in ledger updates (cryptocurrency payments, etc).