DNA, or deoxyribonucleic acid, is an extremely complex molecule that serves as the "master molecule" of organic life by encoding for proteins that form cell structures. DNA molecules are made up of two intertwined chains of simpler molecules called bases, assembled on rails of sugar and phosphate molecules like rungs on a ladder.
Four different kinds of molecules form the rungs of the DNA ladder: adenine, thymine, guanine, and cytosine, which are typically referred to by their first letters, A, T, G, and C. In the DNA molecule, A always pairs with T, and G always pairs with C. DNA molecules replicate by "unravelling" their rails. Each rail keeps its associated base. Complementary bases (A for T, G for C) attach to the free bases until two new strands of DNA are created.
Unique to each individual, DNA is often used to identify individuals. By the 22nd century, Vulcans were obtaining DNA samples from every infant Vulcan and storing the DNA sequences in a massive database known as the Vulcan Genome Registry. In 2154, a Romulan back faction of the Vulcan High Command planted a DNA sample from T'Pau on the remains of a bomb exploded in the United Earth embassy, hoping to frame her and the Syrrannite movement for the bombing. However Doctor Phlox quickly determined that the DNA had been purposely planted on the bomb fragments. (ENT episode: "The Forge")
The result of this DNA being spread across the galaxy is the humanoid form itself. This DNA creates a tendency toward a specific external form. The similarity created allows even for inter-species reproduction between species that evolved quadrants apart, however the internal workings even down to the base element of the blood can still be quite diverse. (TNG episode: "The Chase")
The following year, former Ferengi DaiMon Bok somehow resequenced the DNA of Jason Vigo in order to convince Captain Jean-Luc Picard that Vigo was in fact his son in order to take revenge on Picard over the loss of his son. This elaborate deception was uncovered by Doctor Beverly Crusher when Vigo began suffering seizures resulting from the DNA resequencing, and also due to the fact that Vigo suffered from Forrester-Trent Syndrome. Forrester-Trent Syndrome was inherited from one parent, and neither Picard nor Miranda Vigo had the condition. (TNG episode: "Bloodlines")