Looks like i'll have to make the lengthy explanation right now, i'll try to trim it down ~_~
Ok, Sharingan, or the Copy Wheel Eye, is one of the three great doujutsus, and the bloodline limit of the Uchiha clan, it's able to perceive high movements that are above the normal eye's ability to see, it does this by slowing down the speed in your eyes, basically you see movement happening slower than it normally is, thus being able to track it, and predict it according to the movement pattern, however being able to track it is something and being able to react to the fast movement is another, for that your body needs to be able to keep up with the speed of the body movement your tracking, the Sharingan only gives you more reaction times, it doesn't grant you more speed, the perceiving ability is unlocked in the 1 Tomoe and it's considered the sole 1 Tomoe ability, other than being able to break through visual D-rank genjutsu, which brings me to my second issue, the Sharingan doesn't allow you to break all kinds of genjutsu, not meaning different ranks, but genjutsus have different kinds, ones that affect your hearing, others that affect your sight etc, Sharingan is only able to pierce through visual genjutsu techniques, meaning the genjutsu that affects your eye sight only, if you are encountered by a genjutsu that affects your hearing the Sharingan won't be able to pierce through it, ofcourse piercing through visual genjutsu is also limited up to ranks, 1 Tomoe D-rank, 2 Tomoe C-rank, 3 Tomoe B-rank, Mangekyo A-rank, all visual genjutsu, however knowing that you are in a genjutsu and breaking through it and completely irrelevant, it's a basic Sharingan ability to be able to identify yourself when you are inside a genjutsu, this is done by seeing your own chakra's irregularity since it's an influence of genjutsu, it makes you able to know that you are inside a genjutsu once you are struck by it, then you attempt to break it, either through the Sharingan with B-rank and below visual genjutsu, or by normal means.
Moving on, the most distinguishable ability that the Sharingan has is the ability to see the chakra flow, which is unlocked when you achieve 2 Tomoe level, the Sharingan gives each person a different chakra color, distinguishing them by their chakra color, when you advance in Sharingan to the 3 Tomoe level, your Sharingan will have a better clarity of perception that it would be able to differentiate between the basic 5 elemental chakra just by seeing them, as it would be able to see their unique features, for example wind chakra is composed of very thin blades, fire chakra is extremely hot, lightning chakra makes rapid vibrations, earth chakra is very dense, and water chakra has a certain flow, and the Sharingan cannot see through chakra, meaning if you released a fire technique for example you wouldn't be able to see past the chakra, no matter how thick or thin it was, you'd be able to see through non chakra infused surfaces, but it would be right below the surface not very deep within it, like 2 meters deep max, Sharingan isn't like Byakugan.
Then we have the predicting and copying ability of the Sharingan, the Sharingan like it predicts Taijutsu it's able to predict ninjutsu techniques, however you must be able to see your opponent's hands to be able to identify the techniques accurately, you'd be able to predict it through the handseals they make or if you've encountered the technique before in case of custom techniques, you can't just predict a technique you don't know, you can only know the element that the technique is from when you have 3 Tomoe, but not the specific techniques, given this any cannon technique you know you'd be able to identify providing that you see the user's hands, you can't copy CJ techniques too, so it would be unique to their users.
This is critical information to know about the Sharingan since most new Sharingan users encounter these kind of situations, i've encountered them myself, so you should read it all, then if you have any question ask me ^^