Twins can be fraternal or identical.  Fraternal twins occur when separate sperm fertilize two eggs.  In this case, the amount of DNA in common between the twins is comparable as for siblings in general.  Identical twins occur when a single egg is fertilized with a single sperm and very early during development the egg splits into two separate eggs.  These two eggs contain the exact same genetic potential.  By comparing the level of genetic similarity between twins it can be determined if they are identical or fraternal.  It is not necessary to analyze the parents in this type of test.