An excellent question & an excellent answer by our Muslim brothers , but one thing i'd like to add to the answer , that everything is mentioned in the Quran , as Allah s.w.t promoses in the ayats of Quran, but these ayats are best explained by none other than our Holy Prophet Muhammad (S.A.W) p.b.u.h , so in order to understand the Quran & follow it perfectly you have to follow the Prophet I.e the Hadiths , you have to follow the interpretation
For e.x - when I was in school & used to study english literature we had to read story books & in orde to understand the story properly & to know its summary we had a guide book. So the story book wasn't incomplete without the text book & complete by it's self , but in order to understand it better we needed the guide book , in the same way the holy Quran is complete by it's self but we need the Hadiths to understand it fully & properly