Supplements as the name suggests are substances that supplement the nutrients that you get from food. They are not to be used as a substitute for proper food. They are widely available and are mostly taken for health reasons.
In the United States, dietary supplements are substances you eat or drink. They can be vitamins, minerals, herbs or other plants, amino acids (the individual building blocks of protein), or parts of these substances. They can be in pill, capsule, tablet, or liquid form.