DK DPS Armor / WeaponsFollow

All right. I've heard a lot of people talking about DK's rolling on Tanking armor / weapons, but I have a question about this. What is the true difference between Tanking armor / weapons and DPS armor / weapons? I honestly don't know, as I've never played as a melee fighter before. Is it just Str. and Sta. that I need to focus on, or are there other things that I need to be on the look out for as well? I've currently got the cobalt set, as I heard that it was a good set for DK's, but I'd like to know so if I'm in an instance as DPS I don't get in trouble with the tank for rolling on a Tanking item. Any help is greatly appreciated.