What are dementors?

Answer 1

dementors feed of fear and they bring back your worst memory's for example Harry's was his parents dying, if the dementor is going to kiss you then it means they are going to suck out your soul which means you will dye, the only way you can fight a dementor is thinking of your happiest memory and using the spell expecto patronum and this produces a patronus which will be an animal of some sort Harry's is a stag.

Answer 2

Dementors are non-beings, Dark creatures, which feed off human happiness, thus causing depression and despair to anyone near them. They can also consume a person's soul, leaving their victimes in a permanent vegetative state.

The Ministry of Magic employed Dementors as the guards of Azkaban prison, until mid-1996, when Lord Voldemort was sighed at the Ministry, and their defection to his side was realised.

There are certain defenses one can use against Dementors, specifically the Patronus Charm. Dementors hold no true loyalty, except to whomever can provide them with the most people to feed off. They cannot be destroyed, though their numbers can be limited.

- Taken from harrypotter.wikia.com