There are different kinds of DNA testing that you can do for genealogical research. The first is Maternal line (mitochondrial dna) testing. Anyone can take this test, as everyone carries their mothers mtDNA, but it is passed on by the women in your family, so that's the line it will trace. The next kind of testing is Y-DNA, which follows the Paternal line. As it tests the Y chromosome, only men can take this test (since women don't have a Y chromosome). Finally there are autosomal tests, which is designed to find relatives that share ancestors within 5 generations. This can help you find living cousins that share an ancestor on either side of the family (you'll probably have to do genealogy work to figure out which ancestor you share). This is the newest test they have made, and hopefully it will get refined as time goes on.
All DNA testing is best done to validate family relationships and origins. Although, it can help point you in the direction of where to look for relatives, or countries you may not know you have connections to. It's all very interesting, and still fairly new science. But definitely worth checking out if you have the time and money to do it.