The essentials
Ethereum executes programs called smart contracts. Transactions consume computational resources measured in gas, and users pay fees in ETH. A successful transaction can change balances or interact with a decentralized application.
What to consider
The base fee varies with demand and is burned; a priority fee incentivizes inclusion. A reverted transaction can still consume gas because validators performed computation. Layer-2 networks offer different costs and security assumptions.
Put it into practice
Check the destination network and preview the transaction. Keep enough native ETH for fees and never assume a token balance can pay gas.