Well, the sharingan is a dojutsu technique of the uchiha clan. The sharingan allows you to see much better than the normal eye, and percieve motions which you would have missed otherwise. The sharingan also allows you to slow down, and analyze an opponents attacks by watching what signs he weaves and how he moves if he is using taijutsu. The sharingan is also seperated into different tomoe, which have varying degrees of perception and clarity.
1 Tomoe: This basically just looks like an extra pupil or tomoe in the eye. The entire eye becomes entirely red with one black circle, and within the circle is your pupil while on the circle is the tomoe. This tomoe allows the user to percieve basic motions. This includes most taijutsu techniques, and movements that would normally be invisible to the naked eye. You can also see chakra at this level, and can distinguish different chakra colors. This tomoe can see through D rank and below visual genjutsu.
2 Tomoe: At this rank it looks the same as the first tomoe, but with a second tomoe on the circle. At this point, you can see even faster movements. You also gain the ability to track handseals, and copy them along with other techniques. You can also see through visual Genjutsu C rank and below. You also get better clarity,and perception.
3 Tomoe: Looks the same as the other two, but with an extra tomoe in the circle (so it has 3). You can also see better, and better percieve motions. You can also see even faster movements, and like the others copy hand seals nin gen and taijutsu. You can also hypnotize a target two ranks below you, and can see through b rank and below visual genjutsu.