The Rasengan is an A-rank technique and therefore extremely difficult. This technique was invented by Yondaime Hokage and has undergone 3 years perfected by him. Since you need for this technique a very good Chakrakontrolle, very few dominate the Rasengan. The Rasengan to be exact, is / was only five people dominates (more on that below).
The Rasengan is a special technique, because it does not require finger sign to create the Rasengan. In addition, the Rasengan duplicated his strength, once you've created it, without which one must continue to give Chakra added. The "normal" Rasengan is an incomplete technology per se. Minato Namikaze had planned with the Rasengan to drive both the power of the normal chakra and the force of Naturchakras until the maximum and combine. This, however, he succeeded only with the normal chakra, since he was not able to combine the two Chakraarten together. Rasengan means "scroll ball". In Naruto Uzumaki Chronicles the Rasengan is called "Power Strike" or "power stroke". Apparently Minato Namikaze from Bijuu Dama sought inspiration for the Rasengan.