Ladders are for those players that are looking to compete against other players in a ranking system. War Of Ninja has mutliple ladders each with its own focus.
The Ranked League is a points ranking system with a new seasonal reset every quarter of a year. You gain points by winning and lose points when losing battles. The amount of points you gain or lose depend on the expected skill level.
The Clan Ladder is a ranking system based on reputation. You gain small amounts of reputation by winning battles against the AI, but more importantly, Clans can war other clans to claim each others reputation!