the temperature you store DNA at will depend upon the buffer and what you want to use it for. If you have long genomic DNA and want to use it for southern blots, so you don't want to shear it by freezing, you store it at 4C (the fridge) in a buffer like TE to prevent hydrolysis. If it has to be in plain water, ie, because you are pcring it and don't want the EDTA messing up your Mg ion concentration, you store it at -20 (the freezer). In fact most of the time it is stored at -20 if it is solution. If it is plasmid you can dry it and store it at room temp.