A smart contract is a program that is stored in the blockchain to facilitate transactions once two ( or more ) conditions are met.

In the case of NFTs, the smart contract proves that your wallet (the creator) has the NFTs full ownership and the buyers wallet has the funds to buy that particular NFT and only once this two conditions are made the NFT will be transferred from the creators wallet to the buyers wallet.

The smart contract follows simple rules such as "if/when...then..." so for example "IF this person has 1 ETH in their wallet THEN transfer this NFT". This transactions are verified by a network of computers that verifies that this two conditions are met.

Once the conditions are met, the smart contract is executed immediately. Given that there is no third parties, the smart contract provides trust so you can see that the information has not been and cannot be altered for personal interest.

For the transactions to take place they need to go through a network of computers that all work together to validate each transaction. This is why this transactions need GAS, which is just a term to determine the cost of the transaction to be validated. ( https://help.nft-generator.art/en/article/what-is-gas-il4hxp/?bust=1645715445251 )
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