Base pairs are held together by hydrogen bonds. Depending on the nucleotides that make up the base pair, there are either two or three hydrogen bonds. Hydrogen bonds are not actually bonds, but significant attraction forces between positively charged hydrogen atoms of one molecule and a negatively charged atom of another molecule.
All DNA base pairs consist of two nucleotides. There are four possible nucleotides for each spot in a pair, meaning that there are 10 possible combinations for a pair in total. The four nucleotides are adenine, cytosine, thymine and guanine, which are typically represented by the letters A, C, T and G, respectively.