The most common is the representation of the base pairs as letters - an adenine nucleotide is abbreviated as A, guanine as G, cytosine as C, thymine as T, and in RNA, uracil as U.
Additionally the positions of the carbons in the ribose sugar that forms the backbone of the nucleic acid chain are numbered as follows:
Since the hydroxyl group attached to the 3' carbon of one base attaches to the phosphate group attached to the 5' carbon of the next base, 5' and 3' are often used to indicate the polarity of a DNA strand, e.g. "5' to 3'" and "3' to 5'". This is especially important as regards polymerization, where polymerization enzymes move exclusively from 5' to 3' along a DNA strand.