Ninjago is a toy line created by LEGO with a simple story behind them in order to sell them.
Ninjago isn't the first Ninja theme, as in 1998 and 1998 there was LEGO Ninjas, which featured ninja minifigures fightingb against samurai. It was short-lived though, with only thirty or so sets.
When Ninjago began, it featured of course the collectible Spinjitzu smalls. They consist of a simple spinner, blockers, cards, a character, and weapons. The Spinjitzu Starter Set housing Jay and Frackjaw and the arena to play Spinjitzu in. In the story Spinjitzu is a legendary art created by the first Spinjitzu master.
Other sets in that winter were the oriental dojo sets, those being the dojo, skeleton bowling, and arena. These have the functionality of creating a ring. Also for the ninja were two training posts in the forest, and the great Ice Dragon ridden by Zane. In the story the dojo is where Kai, Cole, Jay and Zane learn Spinjitzu from the great Sensei Wu in order to find the Golden Weapons before the Skeleton forces.
The Skeleton armada consists of an ATV, the Skull Motorbike, and the Turbo Shredder. They are all driven by skeletons. In addition to that there is Lord Garmadon's Dark Fortress with the prisoner Nya. These sets filled with Skeletons and are the main antagonistic sets.