DNA is a very large molecule, made up of smaller units called nucleotides that are strung together in a row, making a DNA molecule thousands of times longer than it is wide.
Each nucleotide has three parts: a sugar molecule, a phosphate molecule, and a structure called a nitrogenous base. The nitrogenous base is the part of the nucleotide that carries genetic information, so the words "nucleotide" and "base" are often used interchangeably. The bases found in DNA come in four varieties: adenine, cytosine, guanine, and thymine—often abbreviated as A, C, G, and T, the letters of the genetic alphabet.
A DNA molecule is a double helix, a structure that looks much like a ladder twisted into a spiral. The sides of the ladder are made of alternating sugar and phosphate molecules, the sugar of one nucleotide linked to the phosphate of the next. DNA is often said to have a sugar and phosphate "backbone."
Each rung of the ladder is made of two nitrogenous bases linked together in the middle. The length of a DNA molecule is often measured in "base pairs," or bp—that is, the number of rungs in the ladder. Sometimes, this unit of measurement is shortened simply to "bases."
http://www.genomenewsnetwork.org/resources/whats_a_genome/Chp1_4_1.shtml