The Sharingan Contact Lenses which literally translates to "kaleidoscope copy wheel eye" is known as the "heavenly eyes that see the truth of all of creation without obstruction". It has only been awakened by a handful of Uchihas and by only one non Uchiha -- Kakashi Hatake.
In order to activate this advanced Uchiha technique the user has to undergo or suffer from an extreme pain of loss. Specifically the user must witness the death of someone either very close to them -- like a best friend, which was the case of Itachi -- or the death of a loved one. And of course in order to gain hastened power this requirement has been manipulated in the past by Uchiha clan members killing their own kind in order to expedite the process.
Unlike the sharingan the way the mangekyou looks will vary from individual to individual; and can take on a completely different look or shape given the person who unlocks it. Of course some characteristics remain consistant across every wielder -- such as the colors black and red, along with all wielders eyes resembling variations of pinwheels.
In terms of abilities it can use all of the generic priors such as genjutsu, enhanced perception, etc, however it can also unlock unique and powerful abilities given the wielder. It can also unlock certain powers in both eyes or different ones in each eye -- such as in the case of Obito and Shisui.
Some things people may not know is that techniques of one eye can be transfered into another, along with all of the power of the other eye if the user wishes. Something more advanced can be accomplished like when the user programs a technique to activate or react at a certain time in the future. And this time of programming can even activate after the wielder dies.
However, all of these great abilties do not come without a cost to the user. Continous use will put a large strain on the user's eyes and body along with draining a lot of chakra at the same time. Over the long haul the effects end up becoming much worse; where long term use slowly eats away at the user's eyesight until he is left totally blind and helpless. With the only way to circumvent this terrible fate is by aquiring another Uchiha's eyes and transplanting and claiming them as your own.
Making it even worse is the fact that the eyes have to be very similar genetically or else the body may reject them and the whole process becomes pointless. In order to get this right the eyes typically will have to come from a relative, sibling, or somone with strong blood ties to the new owner. And at this after several days to sync and heal with the new recipient the Eternal Mangekyou is born. The Eternal version boasts incredible power through the fusioning on abilities of both owners, along with visual differences of the actual eyes now being a mix of both. Currently only Sasuke and Madara have been able to unlock the Eternal versions throughout their clan history.
Itachi gained his after watching his best friend Shisui jump off a cliff which then enabled him to use the powers of Amaterasu, Tsukuyomi, and Susanoo. Obito gained his after witnessing the death of his friend and love Rin Nohara die at the hands of his other best friend Kakashi. In this instance they actually both unlocked it at the same time since they both shared one of Obito's eyes. Both he and Kakashi were granted the ability known as Kamui.
Madara unlocked his roughly at the same time as his younger brother Izuna; as they were the first of the clan to wield it -- he was granted the ability to summon Susanoo in many different forms.
Sasuke awakened his sometime after his brothers death when he finally figured out the real truth behind Itachi's motive and previous actions. He then was able to use Amaterasu along with Kagutsuchi.
Izuna along with his brother, and as previously stated, was the first of the clan to awaken his. Unfortunately they were taken by his older brother when his sight began to deteriorate. Most people believe that they were taken by force from Madara, however, they were actually willingly given up in order to do what was best for the clan.
Shishui Uchiha awakened his eyes at some point in his life through ways that were not shown neither in the anime or manga. With his eyes he was granted the ability to cast powerful genjutsu; the most powerful being known as Kotoamtsukami. With this skill his was able to use a powerful mind control over his enemies that they were not even aware of. When his enemies were in this state they also remained completely unaware of its effects while he was controlling them.
The Sharingan Contact Lenses technique was also incredibly powerful due to the fact that it did not require him to make direct eye contact with his enemies like some other technieques require. The skill ended up being of such a high value that Danzo felt the need to steal it from him. After Danzo had claimed one of his eyes, Shisui gave the other to Itachi before drowning in the Naka River.