A bitcoin fee is a fee which is levied on bitcoin transactions. This fee is collected by bitcoin miners as payment for the role they play in clearing transactions.
Bitcoin users can choose whether or not to pay a bitcoin fee, and how high a fee they wish to pay. The higher the fee paid, the sooner the transaction will be processed because bitcoin miners will have an incentive to clear the transaction.
The inclusion of transaction fees are generally programmed into the source code of altcoins as well. Depending on the unique attributes of a cryptocurrency and on demand for transaction by its users, transaction fees may be significant or negligible.