What is meant by democracy and explain the merits and demerits?

Democracy is a system where all the citizens get a vote, and things are decided that way. "Citizens" may be a distinct category from "the population", such as in slave societies, or societies where the vote is restricted to certain classes.

The merits are that it keeps the many from being ruled by the few. The demerits are that the few are then ruled by the many.

It has been truly said that a lynch mob is democracy in it's purest state. 99 men are voting for the hanging, one man is voting against the hanging. The "ayes" have it.