ETH addresses are hexadecimal like: 0x123AFD... The same addresses can be owned by a wallet or by Smart contract deployment. So you can send and get balance of both. Basic functions are:

• address.balance: total weis in account (1 ETH == 10^18 weis).
• address.transfer(amount): send an amount in weis, raising an exception if error.
• address.send(amount): returns true if success instead of raising an exception.

Transfer/send costs are 2300 in gas. Global objects in a contract are:

• msg.sender: Sending address
• msg.value: weis sent
• now: current timestamp

Functions or addresses should be marked as PAYABLE to receive ETH.