This jutsu allows the user to transform either themselves or another object into the appearance of another person, animal, or object. This is one of the basic jutsu taught at the any Ninja Academy. Fundamentally, all ninja know this jutsu. The transformation technique is considered to be among the more difficult E-ranked techniques, since it requires constant emission of chakra while mentally maintaining the form. On top of that, the user would be, most likely, interacting with the environment. This puts much mental strain on the inexperienced ninja.
Tsunade of the Sannin possesses a more advanced version of the Transformation Technique, where she disguises her 50-year-old self as younger versions of herself. Her technique is unique as it seems to be permanent, nor does she need to exert herself to maintain it for a long time, whereas a normal transformation would be dispelled when the user is injured. This implies that the form is a physical transformation and that it only breaks when Tsunade is low on chakra. Ami Hyuga] also has this ability to appear as an innocent little girl.
Tenka Nihisaki also possessed a more advanced version of the Transformation Technique, which allowed her to transform herself into anything she could imagine as long as it was human. This allowed her to disguise herself as a young Genin despite being in her late thirties. Kin could also use this technique but he didn't need a tailed beast chakra like how her technique was powered by the chakra of the Eight-Tails allowing Tenka to use it almost indefinitely, it could also simulate aging as shown with Tenka's genin disguise developing through out the year in which the technique was active.