Okay the Sharingan in a nut shell....oh god (trying to do this without narutowiki and lookin at other threads as this will be more honest and good for me in the long run)
Well it was one of the most sought after doujutsu if not the most sought out. It orginates from the Uchiha clan. When the sharingan first activates it usually does not automatically activates into its most normal form of the three tomoe. It usally progresses from 1, 2 and 3 tomoes. Tomoes refers to the amount of pupils circulating around the eyes. Uchiha's are not born with their sharingans but are usally required to experience a tramatic event then it is awakened. For example Sasuke's sharingan became awakened as he was fighting Haku, he gained more tomoes in his fight with Naruto. Other noticeable people who had the sharingan but were not uchihas were Danzo and Kakashi.
The sharingan regardless of its tomoe helps the user track an opponents movements and jutsu. The more tomoe's you have the easier you can track their jutsu/movements. When the three tomoe is activated copying an opponents jutsu is also available. Sasuke did this to an extent but Kakashi becam the most known for copying over 1000 jutsu with his sharingan. In our RP this ability is heavily restricted as you cant copy an opponents customs and can only copy jutsus you meet the requirement for. The sharingan allows the user to see chakra but not chakra natures. This can help in breaking out of genjutsu, preparing counters, or noticing others within a genjutsu. The sharingan has its own special form of genjutsu that focuses solely on eye contact. Looking into the eyes of a Uchiha or sharingan user is dangerous as the users of the sharingan are usally adept in their own special sharingan genjutsu. As noticed Gai fought in a style focusing on the lower half of the body of Itachi because he did not want to risk being placed in a genjutsu.
Mangekyou Sharingan
This is a stage beyond the three tomoe and much more powerful. Genjutsu from the Mangekyou is extremely powerful and almost unbreakable unless you have your own MS activated. In this stage the user usally attains some special power different from those of the normal sharingan such as amaterasu, kamui, tsukiyomi, etc. It is awakened by the death of someone close to you causing you to feel the emotion and awaken the new sharingan. Continued use of the MS causes eye sight deterioration. Use of this is extremely chakra draining and limited in its use. Incomplete Susano'o was seen to be availiable in this stage
EMS
Very few have achieve ths and this can be achieved by transplanting the eyes of another Uchiha(probably a blood relative) to restore your eyesight to a point you eye won't deteriorate and their power increases yet again. Madara and Sasuke have been shown to have the EMS and use a complete Susano'o. This is the last and most powerful stage of the sharingan
Know I missed some stuff but thats the best I could do for now sorry.