iPoker is simply a poker network, what this means is they operate a central poker room and they offer online casino and sportsbook the opportunity to run their own branded poker rooms using players and prizepools from this central poker room. It allows sites such as Ladbrokes, 365bet or betfred to instantly setup a poker room without the headache of having to develop their own software and built their own player base. Because the player base is shared among all the poker rooms it creates a huge base of players and allows for many more cash games to be running 24/7 and tournaments with bigger prizepools.
These mini poker rooms are known as skins, for example ladbrokes may run an ipoker skin which is a poker client branded with the ladbrokes logo etc. Customers login to this ipoker client using their existing ladbrokes username and password.
iPoker is the largest poker network on the planet. Although they aren’t the biggest poker room by a long way (this title belongs to pokerstars). Other networks include microgaming, more known for their online casino which has been running for 20 years now. They power sites like stan james and 32red. Ongame, a rather shoddily and badly designed poker network, the owners of this network went on to buy pokerstars recently.
The downside to running a skin on a poker network rather than your own system is that you have little control over what happens. i.e. if you want to introduce a new game format then you have to get ipoker themselves to introduce it and they have to keep 100s of other poker skins Happy at the same time. also ipoker set rules on what sort of promotions and rakebback each site can offer, whereas an independent site can do whatever they want.