An Ethereum wallet is an app or device that holds the keys to an account on the chain. The ETH and tokens stay recorded on the network itself; the wallet shows them and signs transfers. A new wallet displays its seed words once, and those belong on paper, kept apart from the phone. An ETH wallet address starts with 0x followed by forty hexadecimal characters. Several compatible chains use exactly the same format, so a 0x address alone does not reveal which network a transfer will travel on. The network name has to match on both sides.
The Ethereum network measures the work behind each transaction in units called gas. Ethereum gas fees are paid in ETH, and the total depends on how much gas an action uses and on the price per unit at that moment, which climbs when the network is congested. A plain transfer needs less gas than a call to a complex contract. Wallets show an estimated fee before anything is confirmed, and on a busy day that fee can exceed a small transfer. A transaction that runs out of gas fails and still pays for the gas it used up.