Your DNA is essentially a set of instructions. It is made up of genes, which are the molecular unit of heredity. Therefore, genes decide your biological traits, such as eye color.
Genetic switches determine which genes should be expressed. For example, a heart cell will express very different genes than a brain cell. When "turned on", these genes are transcribed into single-stranded RNA.
RNA is then translated into proteins, which play a variety of roles in the body, such as catalyzing reactions, replicating DNA, transporting molecules, and much more. Essentially – we all are just manifestation of proteins working together.
Under certain physiological conditions such as stress or disease, a specific protein may be overexpressed or underexpressed. This is the current basis of a biomarker, which is a measurable protein whose altered expression indicates that an individual has a disease. However, in addition to changing the level of protein, diseases also modify the structure of proteins.