DNA is made up of many nucleotides put together. Each nucleotide is made of a phosphate, a sugar, and a nitrogenous base. In DNA, the specific sugar in the nucleotides is deoxyribose (which makes up the 'D' in DNA). When nucleotides are linked together, the phosphates and the sugars form the 'backbone' of the DNA molecule. They alternate along the sides of the molecule, or if you think of DNA like a ladder, along the sides of the ladder. The nitrogenous base of one nucleotide attaches to the nitrogenous base of another nucleotide by hydrogen bonds to make up the rungs of the ladder.
One of four different nitrogenous bases will be on any given nucleotide: adenine, thymine, guanine or cytosine. In the DNA molecule, adenine always pairs with a thymine, and guanine always pairs with a cytosine across the rungs of the ladder. You can remember it by the letters with straight sides go together (A with T) and the letters with curved sides go together (C with G). The order of the bases along a strand of DNA encodes the instructions to make proteins, which go on to be involved in all life functions. This is how DNA is considered the code of life.