Gel electrophoresis is a method for identifying the different molecules in a mixture by placing them in a gel in an electric field. Molecules such as proteins and DNA are negatively charged and move towards the positive pole.
Different proteins and different variants of the same protein have different net electric charges, because they have different amino acid compositions. If we place a sample of proteins (with the same molecular weight) in an electric field, those with the largest electric charges will move fastest.
The importance of the method is that it can reveal different variants of a particular type of protein. The picture opposite shows a sequence of DNA which has been sequenced using the gel electrophoresis method.