Diet is also a pretty big canvas to paint on. In the most general terms, your diet is what you eat - as in the specific foods you eat. There are tons of "diets" out there that numerous people "follow" and try to get others to follow. Books on diets have been a staple of the publishing world forever it seems. There are far too many diets to list - although Wikipedia does a pretty good job of it.
So let's forego the pros and cons of the Paleo diet vs the DASH diet or any other sets. We humans are pretty good at rationalizing whatever set of rules we chose to follow; be it politics, education, fitness, and especially diet. So I'll try to leave that up to you and your resident "expert" - be that your doctor or your opinionated uncle.
But let's DO talk nutrition a bit. No matter what specific foods you eat, you eat them to get your body the energy it needs and the nutrients (macro and micro) it needs to continue running properly. Your body is a very complicated organism with lots of working parts - muscles, organs, blood, hair, enzymes, and so on. Those nutrients from food are necessary for your metabolism which is the whole set of chemical reactions/processes that you need to stay alive.
The OVER and UNDER: We also know that you can over do it. Your body will do an excellent job of energy storage in the form of fat and this can lead to being overweight. You can also under do it and be underweight. There are guidelines to what "just right" is when establishing your weight. No matter where you fall on the weight spectrum you must keep yourself properly nourished. Malnutrition is the state in which your body is NOT getting the proper set of nutrients to maintain all your biological processes. Malnutrition can, and often does happen even in cases where one's caloric intake is sufficient. Anyone can be malnourished, regardless of weight - the nutrients just aren't all there. Then there is undernourishment. Being undernourished means that you are are not getting enough food (fuel) to properly maintain your metabolic needs. Your caloric intake is too small to match your needs. While many people around the world are struggling to just meet the minimal requirements to avoid malnutrition and undernourishment, we here in the USA are easily on the other end of that spectrum. Our own Center for Disease Control and Prevention estimate that 70% of all adults in the USA are overweight and half of those (35%) are considered obese. Should we be concerned?
Well, we now spend over $351 billion 💰 annually on 💔 cardiovascular disease (CVD). It is the leading cause of death in our country. So to answer the question... yes, we should be concerned. Want more? Follow that link above and read. Yikes. See how easy we can get off track and dive into more pressing issues? There IS a part of the upcoming chemistry that ties into the whole heart health thing - which is why I mention it here. Anyway, for now, I'll right the ship and get back to the chemistry storyline.
Macronutrients are the ones listed first on a "Nutrition Facts" label found on most every food that is packaged here in the US. They are carbohydrates, fats, and proteins. Those 3 classifications of compounds/foods are our macronutrients. The "bulk" of all our food products - they provide energy and the main building blocks for our bodies to function. Their amounts are generally given in grams or even "servings" which are then defined as a certain number of grams for that food item. All those grams of food also add up in the amount of energy they provide. That energy is measured in Calories (note the capitalization - I'll explain elsewhere). We (the US) are set in our ways of not embracing SI units of energy (the joule J, or kilojoule kJ).
Vitamins and minerals are our micronutrients. We just need a tiny amount of these compared to macronutrients. These tend to have quantities of milligrams (mg) or even micrograms (µg or mcg) - certain key micronutrients are listed on the labels of our foods along with the macronutrients. Although the quantities are small, they are still quite necessary to maintain one's health.