DNA (deoxyribonucleic acid) is the organic molecule that stores the information that allows the synthesis of all the components of the organism. It could be equated to the organism's manufacturing manual. In the human species, most DNA is found inside the cell nucleus. With exceptions such as red blood cells, all cells in the organism carry the same DNA, and the differences between cell groups are due to the regions of that information that they encode and read. It is also responsible for hereditary transmission, that is, the traits and characteristics that individuals pass on to their offspring.
This is what allows a study of the DNA of the entire individual to be made from cells found in saliva. It also means that, from one individual, predispositions for diseases and traits can be known in other blood relatives.
Nucleotides are the molecules that make up DNA. DNA is actually a polymer of nucleotides, a chain of nucleotides forming the famous double helix. There are four types of nucleotides in DNA: adenine, cytosine, thymine, and guanine. Depending on the order these four nucleotides follow, the information they transmit to the cell is different. If DNA is the manufacturing manual, nucleotides are the letters in which the manual is written.
Precisely, SNPs are changes of one nucleotide for another, for example, changing an adenine for a thymine in the DNA chain.