If you have ever taken an economics class you are probably familiar with the Pareto Principle better known as the 80/20 rule.
The Pareto principle (also known as the 80-20 rule, the law of the vital few, and the principle of factor sparsity) states that, for many events, roughly 80% of the effects come from 20% of the causes.
Although this rule is more commonly seen in the business world, fitness is slowly making its way into the club. 80% of what you look like and weight management is due to nutrition and 20% is due to exercise. Yes, I know it doesn't quite fit the pattern but you get the gist of it. What you eat matters, as much and if not more than how you work out. I often hear of people busting their ass in the gym and going home to a pizzas and pastas and then complaining about why they don't see any results. Eventually they swear off the exercise because it's not working and send themselves into a downward spiral of poor nutrition and poor health. Within the same 80/20 club, nutrition experts agree that it is not important to be 100% to achieve results, instead eat nutritiously 80% of the time and then allow yourself to indulge 20% of the time.
But don't take my word for it, experiment for yourself. If you exercise like a mad man and eat sub optimally, try cleaning up your diet. If you are on point nutritionally and exercise minimally, try letting loose on your diet (for experimental purposes only!) and exercise more. I am a big advocate for exercise, it has a million benefits, and I couldn't every imagine giving it up, but when it comes to weight management its more than just exercise it's about nutrition.