what is a comit  and can it be harmful  to us

Comets basically are cosmic wanderers which move around in elliptic orbits about the Sun. They are primarily composed of gas, dust and ice which are held loosely together under gravity. Most of them are located in the Oort Cloud in the far reaches of the Solar System. 

A few examples of famous comets include Halley's Comet, Hale Bopp, McNaught etc.

Yes they are potentially harmful (even more than asteroids or meteors) because they move with greater speeds (and thus possess more energy) moving through space.