The Rasengan is a technique invented by the 4th hokage after he has observed the Bijuu Dama. The Hokage studied it for 3 years, taking the power of this technique to newer levels, just by relying on shape manipulation. The shape manipulation shown in the rasengan is so great that it can only be done by master control over chakra, but Minato died before including the last part which was supposed to be Nature manipulation, but this was later accomplished by his son, Naruto. The rasengan was created as an ultimate meele technique which requires no handseals to perform. Once it is created it becomes self sustaining. The chakra movements of the rasengan are entire unique and it is composed out of many layers of moving chakra in different rotations. The force of the rasenagn comes from the multilayer chakra which tries to dig itself into the opponents body, and then is released into a spiraling after math, which has enough force to destroy not only human bodies, but even far more versatile defenses and materials.
Though Rasengan was firstly created to be combined with elemental natures, but it was too tough of a task to be have been done by Minato before death. Even kakashi tried the very same, but not understanding the true movements of the rasengan he couldn't create something which he wanted, but ended up inventing a new more solid version of the chidori...basically th Raikiri. Though this task was later finished by Naruto as he created wind release: Rasengan and Rasen-Shuriken, an even more advanced version.
Noteable users of this technique are: Minato, Jiraiya, Naruto, Kakashi, Konohamaru.