Both DNA and RNA are composed of repeating units of nucleotides. Each nucleotide consists of a sugar, a phosphate and a nucleic acid base. The sugar in DNA is deoxyribose. The sugar in RNA is ribose, the same as deoxyribose but with one more OH (oxygen-hydrogen atom combination called a hydroxyl). This is the biggest difference between DNA and RNA. Another difference is that RNA molecules can have a much greater variety of nucleic acid bases. DNA has mostly just 4 different bases with a few extra occasionally. The difference in these bases (between DNA and RNA) allows RNA molecules to assume a wide variety of shapes and also many different functions. DNA, on the other hand, serves as a set of directions and that's about all (but that's absolutely necessary!).