ERC20 code outlines six specific functions for tokens, which are
1- Getting the total supply of tokens via the “totalSupply” function.
2- Retrieving the token balance of another account associated with the “_owner” address via the ” balanceOf(address _owner) constant returns (uint256 balance)” function.
3- Sending a specific amount of tokens “_value” to a given address via the “ transfer(address _to, uint256 _value) returns (bool success)” function.
4- Sending a specific amount of tokens “_value” from one token (contract) address to another token (contract) address via the “transferFrom(address _from, address _to, uint256 _value) returns (bool success)” function.
5- Enabling a specific account to withdraw tokens from one’s account repeatedly, while predefining the upper limit for the amount of tokens to be withdrawn with the “_value” parameter. This can be achieved via the “approve(address _spender, uint256 _value) returns (bool success)“. The upper limit for withdrawal, i.e. the “_value” parameter, can be overwritten when the function is recalled.
6- Returning the residual amount of tokens, within the preset amount defined by the upper limit allowed to be spent by the “_spender” to withdraw from the account of the “_owner“. This can be executed via the “allowance(address *_owner*, address *_spender*) constant returns (uint256 remaining)” function.
These six functions defined by the ERC20 code represent cornerstone functionality issues, which include how these tokens will be transferred between different accounts, and how users can retrieve data associated with a given ERC20 token. These group of functions are prescribed to ensure that Ethereum based tokens will function similarly within any part of Ethereum’s platform. As such, all crypto wallets that are compliant with the ether coin will also support tokens based on the ERC20 standard.
Source: https://www.cointelligence.com/content/comparison-erc20-erc223-new-ethereum-erc777-token-standard/