DNA electrophoresis is an analytical technique used to separate DNA fragments by dimension. It is an example of chromatography. Basically, an electric field forces the fragments to migrate in a gel. DNA molecules normally migrate from negative to positive pole due to the net negative charge of the phosphate backbone of the DNA chain. The molecules are forced to pass through the gel, that at molecular length scale looks much like a random, intricated network. Long molecules will migrate slower, because it is easier for them to be 'trapped' in the network, and short molecules will migrate faster.
Once the migration is completed, the fractions of DNA fragments of different length can be seen using a fluorescent dye specific for DNA, like ethidium bromide. The gel will show little,more or less thin bands corresponding to different DNA molecules populations with different molecular weight.
The main DNA electrophoresis technique is Agarose gel electrophoresis. Other techniques used are Polyacrylamide gel electrophoresis, a technique used for proteins but that's handy also for short DNA molecules,and capillary electrophoresis.
