There are two possible answers to this. The one I'm assuming you're looking for is hydrogen bonds.
Hydrogen bonds are the bonds which hold together the double helix shape of DNA. These bonds result from a positive and negative charge drawing two molecules closer to each other. These bonds are generally seen as weak, however, since there are so many of these bonds (as there are so many nitrogenous bases in one strand of DNA), the double helix strand is fairly strong.
