How does gel electrophoresis scientists compares dna samples?

An electric current is passed through the individual nucleotide samples in the wells of a block of agar, and causes them to travel away from the well to either the anode or cathode. 
The individual charge of each nucleotide (nucleotides are slightly negative, and will always travel to the cathode) and its unique abundance in that DNA sample, bands are created by this migration from the well. 
This indicates, again, the unique abundance of that nucleotide in the DNA sample. 
Different peoples' DNA create different patterns of bands, and therefore they can be identified and compared, possibly in the event of a crime.