ERC20 is a token with core functions that behave like cryptocurrencies. ERC20 is likely the most common that are traded but others may have added functionality (and still be compatible with ERC20 if they retain the core functions).
The implementation and functionality of any contract is up to the author and so the functions themselves can do anything the author wants them to, including acting maliciously. By that, it's most important to only transact with contracts that you can know are trusted.
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ERC20 is a token with core functions that behave like cryptocurrencies. ERC20 is likely the most common that are traded but others may have added functionality (and still be compatible with ERC20 if they retain the core functions).
Hope that helps.
The implementation and functionality of any contract is up to the author and so the functions themselves can do anything the author wants them to, including acting maliciously. By that, it's most important to only transact with contracts that you can know are trusted.