Online poker means that players playing against each other, not the casino. The casino just has the function of being a host for all gamblers to play at and they get the commission regardless of who wins and losses.  That's why they say that the house always wins.
That commission is called the Rake. When the pot is made, an amount of money will be deducted and given to the poker casinos.
Time Drop – When new players enter the game, that new players will be charged a fee to have a slot sitting at the table. The amount may be low as $3 per player.
Tournament Fees – When an online casino holds a tournament, the casino will charge an entry fee with a certain percentage of that money for the house.