DNA testing methods vary, but most methods can perform with very high levels of accuracy.
There are many reasons why you might want to get a DNA test. Although we often see DNA testing used in TV forensics shows, and in fact DNA testing is used in real life criminal cases, the most common reason out there for getting a DNA test is to determine paternity. DNA paternity testing conclusively determines if a man is the father of a particular child. DNA maternity tests and sibling tests are also available. For those who are interested in genealogy or ethnic origins, many DNA labs now offer DNA ancestry testing.
How your results will look depend on what type of DNA test you ordered. For example, in a DNA paternity test, the alleged father is either excluded or not excluded. (You will never be told that the alleged father is definitely the father, no one is willing to accept liability for that because there always is that tiny, tiny chance that the test went wrong.) Paternity and maternity tests can provide conclusive results. DNA sibling tests are more complex and the results will be given in the form of a perctentage chance of sibling relationship. DNA ancestry testing results vary by the company that provides them In all cases, it is advisable to do some online research ahead time so you'll know that the test you've ordered provides the kind of results you are looking for.