Network effects mean a product gets better as more people use it. Direct effects (every new Facebook friend makes Facebook better for you), indirect effects (more drivers attract more riders, which attract more drivers), and data network effects (more users improve the model that serves all users) are the canonical types.
NFX, NfX, and a16z have built typologies of 13 to 16 distinct network effect categories. The strongest are tipping markets that go winner-take-most; the weakest are nominal effects that don't actually improve the user experience as the network grows.