DNA (deoxyribonucleic acid) is the molecule that contains the instructions for the development and functioning of a living organism; it can be compared to an instruction manual for building that organism. In humans, most DNA is found in the cell nucleus, with a small amount in the mitochondria.
If DNA is the instruction manual, nucleotides are the letters it is written with: adenine (A), thymine (T), cytosine (C) and guanine (G). Replacing one of these letters with another, for example an A with a G, is a single-nucleotide variant. When this variant is common in the population, it is called an SNP.