Tsukuyomi is one of the most powerful genjutsu in existence. It is said to represent the "Spiritual World and Darkness", the antipode to Amaterasu — a ninjutsu of similar power.
Tsukuyomi requires eye contact to be performed, trapping the target within an illusion completely under the user's control. Itachi's prowess with the technique has allowed him to alter the perception of time with ease within the genjutsu to make a few seconds seem like many days as a means to torture the target. This results in the victim suffering psychological trauma that incapacitates them for a considerable period of time. After Itachi used it on Kakashi Hatake in Part I, Kisame Hoshigaki was surprised that Kakashi's mind was still intact Only Tsunade has been shown capable of curing this psychological damage.
In the anime, during Part I, Itachi's use of the Tsukuyomi typically featured a red moon casting a red light throughout the illusion with a cloudy atmosphere, giving the background a blood-red appearance while the victim within the illusion is in depicted in an inverted grey scale.
Such a powerful technique is not without its disadvantages however. Due to the complexity of the illusion and the quickness with which it is executed, an enormous amount of Chakra is necessary and an added amount of stress is placed on the left eye, leaving Itachi's vision more blurred. Before using it on Kakashi in Part I, Itachi stated that the Tsukuyomi can only be broken by a Sharingan user that shares the same blood as him; which is demonstrated when Sasuke overcame Itachi's Tsukuyomi in their final battle.
If a victim is forced to experience a drastic passage of time in the fraction of a second Tsukuyomi is actually active, the stress on their mind would kill them, even if the Genjutsu was a happy and peaceful one, such as when Itachi uses this technique to kill Izumi Uchiha by having her experience decades worth of time in a fraction of a second.