Definition of fee
The "fee" is a word that means "charge". Fee as in "entrance fee". Even though it is literally translated as fee, the word used in French is "commission". The other term used is rake. All forms of poker have their fees that the house takes. If you are a tournament player, you will see for example a tournament advertised as follows: 10+1. You then know that 10 Euros will go into the main prize pool and 1 Euro will go to the house (casino or online gaming room) as a fee.
Fee or rake in cash games
Fees are also applicable in cash games. In tournaments, usually 10% of the entry fee will be taken as a fee. This is especially true for smaller tournaments. When you have higher buy-in tournaments, it is possible that the fee is lower (lower percentage). In cash game play, the system is different. Many poker rooms will not charge a commission (or fee) if there is no flop no rake. If there is a flop, between 3% and 5% of the size of the pot will be charged as a rake up to a pre-agreed maximum (it depends on the room, but it's rare that a room will charge more than €4 on a pot, no matter how big the pot is).