Alrighty. The Rasengan is regarded as the highest form of Shape Manipulation. It was created by Minato Namikaze, with the purpose of creating a powerful technique that didn't require handseals, an ideal technique for shinobi who wanted to deal damage quickly. However, while the technique didn't require seals, it still ended up requiring a large amount of skill to perform, as the user needed to be able to simultaneously rotate his chakra at high speeds while keeping it contained in a shell, also made of chakra. When the jutsu makes contact with a target, the shell deals a blunt blow and then the roatating chakra explodes out, sending the opponent flying back in a spiral.
Since Rasengan is made of pure chakra, it can be augmented in size, making it bigger. The bigger forms of Rasengan include the Big Ball Rasengan, and the Super Big Ball Rasengan. Rasengan was effectively completed by Naruto Uzumaki when he infused his Wind Chakra into the jutsu, creating the Rasenshuriken.