The curse seal mark is a jutsu created by Orochimaru using the blood of Juugo. Orochimaru tested the seal on ten kids in Konoha, of the ten one survived Anko. During the chuunin exam ach Orochimaru left the mark on the back of Sasuke's neck it would later be reviled as the one of the most power marks, the heaven curse seal this is also the same mark that was left on Anko. The second strongest is the Earth curse seal giving to Kimimaro. The curse seal has two levels.
Level 1: at this level it covers half the users body in a seal that looks like a tattoo. It also seems to double if not triple the users charka level, best example is when the Sound Four took on a squad of Jounin and defeated them with ease after activing the seal.
Level 2: In this level the users physical appearance change this seems to differ upon user. The level of the user chakra at this point has yet to be known.
Being the original source of the curse seal Juugo tought he was the only one that could do partial trasformations untill he fought Sasuke, they are the only two known to have that much control over the seal.
Anko Mitarashi (living)
Mizuki's version of the curse seal was one of Orochimaru's early attemps at creating the seal. It involved mixing herbs and medicines then drinking it. Unlike the curse seal that Sasuke and the others have Mizuki's level one seal does change the physical appearance of the mizuki arms and face making them bigger and giving him tiger strips across them. His level two form changes his appearance with looks like a half tiger half human. The side affect to this was it sped up the users aging and eventually leaves the person weak and decaying.