DNA is a series of Trap Cards that were first introduced in Pharaoh's Servant. The latest is "DNA Checkup" which was released in Crossroads of Chaos. Each "DNA" card depicts a doctor with three eyes and two nurses next to it doing something to a patient.
"DNA" cards focus on changing the Types and Attributes of monsters. This can come in handy if you play cards like "System Down", which banish all Machines. Cards like these are only used in combos (or Side Decks). They also work well in a "Charmer" Deck.