Five Nights at Freddy's is a prequel 

The second game is a prequel because when you start the game Phone Guy is alive. I remember that Phone Guy was killed in the first game. There is more. When fishing night 5 and 6 you get your paycheck dated in 1987. After night 6 there is a newspaper clipping saying that the restaurant is closing down( probably cause of the bite of 87 ) and that they are going to make a smaller Freddy Fazbear Pizza( in which the first game )