What is a bank transaction fee?

Could you please elaborate on what a bank transaction fee entails? I'm curious to understand if it's a fixed cost or a percentage-based charge, and whether it applies to all types of transactions, such as deposits, withdrawals, or transfers. Additionally, are there any factors that can influence the amount of this fee, like the transaction size or the account holder's status with the bank? Lastly, are there any ways to avoid or minimize these fees?