A fee is charged for every transaction made within the blockchain. These fees are calculated in different ways, their height can also be VERY different from one currency to the next. Some coins are very cheap or even free to transfer, while in case of others, the fees can be quite high during the prime time or during period of higher than usual activity.
With some coins, you can agree to a higher fee in order to get priority handling of your transactions. Fees are used to support the community and/or awarded to the miners or node operators.