Essential nutrients are crucial for the proper functioning of the human body. They are classified into six categories: carbohydrates, proteins, fats, vitamins, minerals, and water.

1. **Carbohydrates**: They are the primary energy source for the body, found in foods like grains, fruits, and vegetables. They break down into glucose, which fuels cellular activities.

2. **Proteins**: Essential for building and repairing tissues, proteins are composed of amino acids. They are found in meat, dairy products, legumes, and nuts. Proteins are also vital for immune function and hormone production.

3. **Fats**: Necessary for energy storage, insulating organs, and cell membrane formation, healthy fats include unsaturated fats found in oils, avocados, and fish. They also help absorb fat-soluble vitamins.

4. **Vitamins**: Organic compounds required in small quantities, vitamins such as A, C, D, E, K, and the B-complex group, support various bodily functions, including immune health, vision, and blood clotting.

5. **Minerals**: Inorganic elements like calcium, potassium, magnesium, and iron are critical for bone health, nerve function, muscle contraction, and oxygen transport in the blood.

6. **Water**: Essential for maintaining hydration, regulating body temperature, and enabling bodily functions, water is the most crucial nutrient. It is involved in digestion, nutrient absorption, and waste elimination.

A balanced diet that includes these nutrients is vital for maintaining health and preventing deficiencies.