ERC is "Ethereum Request for Comments" and the number "20" is just signifies the unique proposal ID.
There are concept of ERC-20 - guarantees the token created to have simple transferability among wallets and contracts. There are three functions: ‘transfer’, ‘transferFrom’ and ‘allow’ that must exist in every ERC20 contract. People can simply use ‘transfer’ to send their tokens and if someone wants to spend token reside in a contract, the contract must first ‘allow’ the spender before the spender can ‘transferFrom’ token from that contract.
In additional, ERC-223 has been created with more features/functions. U can find more details about ERC, ERC-20 and ERC-223 here:
https://medium.com/cryptomover/what-are-erc20-and-erc223-tokens-307badcca5ahttps://www.cryptoniam.com/what-is-erc20/Github link:
https://github.com/Dexaran/ERC223-token-standard/tree/Recommended#erc23-token-standard