Fnaf is a horror game in which the objective is to reach the sixth night. There are currently 8 Fnaf games, and it's very possible that UCN is the last Fnaf game.
FNaF you can buy on Steam for:
FNaF 1 - 4.99€
FNaF 2 - 7.99€
FNaF 3 - 7.99€
FNaF 4 - 7.99€
FNaF World - Free
FNaF: Sister Location - 7.99€
Freddy Fazbear's Pizza Simulator - Free
Ultimate Custom Night - Free