This technique is essentially a massive version of the Shadow Clone Technique. Instead of a few clones, it creates a few hundred clones (or a number of equal grandeur). This is considered a kinjutsu, because it can potentially be dangerous to the person performing it, seeing as how all of the clones are given an equal amount of chakra directly from the user. Naruto learned the technique from the scroll of forbidden seals.
Few people are able to use this technique effectively, due to the large amount of chakra it consumes. However, this weakness is not an issue to Naruto, due to his massive chakra reserves. Having such reserves allows him to create hundreds of clones while maintaining a decent supply of chakra in each one. This technique is also used by Naruto to train quickly, since all the clones share their experiences with Naruto, but it causes extreme fatigue with repeated use.
Naruto is shown to use this technique on multiple occasions to form human chains, for both offensive and supplementary reasons.
Sometimes Naruto simply refers to this technique as just the "Shadow Clone Technique", regardless of how many clones he creates.
At the end of Prologue — Land of Waves, Kakashi despite not having enough chakra left to defeat Gatō's thugs, Kakashi uses uses a "Kakashi Version" (カカシVer.) to create several dozen copies of himself as a bluff, scaring them away.
Since Naruto lost right his arm, he was forced to perform this technique with one hand until he gained a prosthetic one. This is done by intertwining his left hand’s index finger with the middle finger to create the necessary hand seal.
Hōki created the facade of using this technique by using a visual projector.