DTube stands for Decentralized Tube. The idea is to get rid of the middle-men that make money on the content production and distribution. DTube allows users to do everything on their own, even if our servers were dead DTube would still work (a few features like search would die, but the rest would work). Also DTube cares about your privacy (we do not collect any single information about our users)

DLive starts with the same letter but I have no idea what their D stands for, as nothing of what I said applies to them.