Different Dimension (D.D.) is a name given to a series of cards all starting with either 'D.D.' or 'Different Dimension'. These cards focus on removing cards from play, often on their demise, thus sending them to a 'Different Dimension' rather then simply destroying the other monster. They can use those monsters to get more powerful or gain special effects. Although they were released in different sets, a large amount of 'Different Dimension' monsters were released in Structure Deck: The Dark Emperor
D.D. Cards are great for cards that resolve on removed from play cards, such as "Golden Homunculus", "Gren Maju Da Eiza", the 'Helios' cards, "Graverobber's Retribution", Caius the Shadow Monarch, etc.
There are also cards that help removing at the same time that represent a defense for you as Dimensional Prison, D.D. Trap Hole, D.D. Survivor, D.D. Scout Plane, etc.
These cards are not without their weakness however. Imperial Iron Wall will effectively shut down all D.D. cards (with the exception of a small few like Different Dimension Dragon.) None are particularly powerful, with their strongest monster, D.D. Survivor having only 1800 ATK Points. It is hard to create a deck filled with nothing but D.D. cards, so in decks these are normally used as a sub-type (mostly in RFP Decks).
Though, if used correctly, the effects of Banisher of the Radiance can be used to drastically improve this archetypes main effect. An excellent counter against Lightsworns