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There are many examples of smart contracts. Probably the most popular are in the crypto currency cryptocurrency space, such as Bitcoin and Ethereum. For example, Bitcoin does have a smart contract, albeit a hard-coded one. In other words, the Bitcoin smart contract defines who, what and how Bitcoin works. Whenever someone initiations a transfer of Bitcoin funds, they are essentially executing a smart contract. You cannot simply modify the Bitcoin smart contract without the majority of the network agreeing your version. The other option would be to fork the network and creating an entirely separate blockchain network. This is similar to how Bitcoin Cash came to be.
Ethereum, on the other hand allow , allows you to run various smart contracts on the same network. TBD
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