WETH is an ERC20 token that is designed to have the same value as ETH, essentially just ERC20-ifying ETH. All ERC20 tokens are essentially just smart contracts (little computer programs that are hosted on and ran by the Ethereum network) that contain the balances of everybody who holds the particular token the contract is for.

ERC20 token contracts have a set of standardised functions (sections of code that must be pr