All transactions must pay a fee to be included in the blockchain. The fee is paid to the miners in order to incentivize them to include the transaction in a block. The higher the fee, the faster miners will include the transaction in a block. Fees are measured based on the data size of the transaction.
A fork is a change to a protocol or a piece of code. There are two types of forks: soft forks and hard forks.
Fungibility is a property of goods which are interchangeable and indistinguishable. If a good is fungible, no instance of the good is preferable over any other.