Your body needs a certain amount of protein per day. Protein has 4 calories per gram so if you eat 75 grams of protein, that's 600 calories in protein. 600 protein calories out of 2000 total calories = 30% of your calories in protein. That leaves you 1400 calories to play with, or 70% of your total calories for the day.
You can choose to go one of three ways with the rest of your calories. You can follow a low-fat diet or a low-carb diet or use a Zone-type diet where you balance fat and carbs.
For a typical low-fat diet, you'd keep your fat at 20% of calories, so 20% of 2000 calories is 400 calories. Since fat has 9 calories per gram, that means 400 calories divided by 9 = 44 grams of fat per day. That means you're eating 75 g of protein, 44 grams of fat and the rest (the other 50% of calories) in carbs. 4 calories per gram of carbs means you'd eat 1000 calories or 250 grams of carbs.
Low-fat diet of 2000 calories = 30% (75 g, 600 calories) protein, 20% (44 g, 400 calories) fat, 50% (250 g, 1000 calories) carbs.
For a typical Zone-type balanced diet, you'd still eat 30% of your calories (75 grams, 600 calories) as protein. You'd eat 30% of your calories (67 grams, 600 calories) as fat. You'd eat 40% of your calories (200 grams, 800 calories) as carbs - usually low-impact carbs like berries, non-starchy veggies, and whole grains.
Zone-type balanced diet of 2000 calories = 30% (75g, 600 calories) protein, 30% (67 g, 600 calories) fat, 40% (200 g, 800 calories) low-impact carbs.
For a typical low-carb diet, you'd still eat 30% of your calories (75 g, 600 calories) as protein. You'd eat 60% of your calories (133 g, 1200 calories) as fat. You'd eat 10% of your calories (50 g, 200 calories) as low-impact carbs, mainly non-starchy vegetables and berry-type fruits.
Low-carb diet of 2000 calories = 30% (75 g, 600 calories) protein, 60% (133 g, 1200 calories) fat, 10% (50g, 200 calories) low-impact carbs.
The amount of protein stays the same in all three because if you do not take in enough protein, your body doesn't get the amino acids it needs to maintain muscles and repair itself. It will get the amino acids by breaking down your own muscle tissue - never a good thing!!
Sorry if all the calculations are confusing. These are only guidelines and these percentages can be tweaked within each diet. For example, some people on a low-fat diet only eat 15% fat, and some people on a low-carb diet only eat 5% carbs.
I personally use low-carb with about 70% fat, 20% protein and 10% carbs.