What is an Ethereum address?

An Ethereum address is a 42-character hexadecimal address derived from the last 20 bytes of the public key controlling the account with 0x appended in front. e.g., 0x71C7656EC7ab88b098defB751B7401B5f6d8976F. Basically, the Ethereum address is the "public" address that you would need to receive funds from another party through the Ethereum network.

What is a zero address?

Remember, the address can either be a contract or an external account. When the to address is the zero-address, a new contract will be created by executing the code in data (this is what is meant by "code that returns the code").

What is a zero-account in Ethereum?

Within an Ethereum transaction, the zero-account is just a special case used to indicate that a new contract is being deployed. It is literally '0x0' set to the to field in the raw transaction.

What does balances [address (0)] mean?

balances is a mapping of address to how many tokens that address owns. balances [address (0)] then means how many tokens the address 0 (0x00000000...00) owns. Although I don't like the practice, some ERC20 tokens consider tokens that are sent to address 0 to be "burned" and thus don't count them in the total.