DNA, or Deoxyribonucleic acid, is the helical macromolecule which encodes the genetic information that participates in cellular growth and development at the molecular level. (Source: R. Gerber, Vibrational Medicine, Glossary).
"DNA is the abbreviation for Desoxyribo Nucleic Acid, which is a compound found in chromosomes and consists of a long chain molecule comprising many repeated and varied combinations of four nucleotides, one of which is the sugar desoxyribose; subdivisions of the molecule are believed to be the genes. DNA is the major repository of genetic information." (PoL)
All life-forms are DNA-based, that is: their genetic template is stored in their DNA.
When it comes to human galactic history, DNA plays an important part too. Several humanoid species that have evolved further that earth humans have, are believed to have 12 strand DNA, instead of the 2 strand helix that we do have now. Several authors, like e.g. Barbara Marciniak, point out that genetic manipulation in the Atlantean era by extra-terrestrials, stripped our DNA down to a two-strand DNA, instead of the 12 strand helix we had before. This was done to slow down the evolution of the Earth human species. In the near future, however, the earth human race would start displaying 12 strand DNA again.