Network games can be played in public or private lobbies, and allow you to play any level of the game with your teammates.
While in Network mode, you can join a random game by selecting Join game or play a specific level by selecting it from the world map. Alternatively, you can create your own lobby and wait for others to join.
You can share your Heart Points with other players, as well as share your money while you are all inside a shop. You can also execute powerful co-op moves which inflict damage on enemies or heal your teammates.
Let us know if you have any other questions!