The way the fingerprint is done is that first a sample of DNA from a person is found. That sample is usually from human cells, blood, saliva or hair (with the root attached). The DNA is then cut up using molecules that cut DNA in a certain way. Then "probes" are added which light up when they bind a particular piece of DNA. In the case of DNA fingerprinting, these particular pieces of DNA are certain sequences that repeat over and over again, but the number of times they repeat is different in different people. This generates an image that looks like a bunch of bars, kind of like a barcode. In a real sense, the DNA fingerprint achieved is a barcode for a person in that it uniquely identifies people.

There are other ways to get a DNA fingerprint, but they all rely on looking at certain places in the genome that we expect to be different and comparing how these places look in different people. So the classic case is if a criminal leaves behind blood, they can do a DNA fingerprint on the blood of the criminal and the crime scene blood and see if the "bars" on the fingerprint are in the same places.