Diplomacy is linked with Charisma.
You can influence others with your tact, subtlety, and social grace. Diplomacy is used to change opinions, to inspire good will, to haggle with a patron, to demonstrate proper etiquette and decorum, or to negotiate a deal in good faith.
A Diplomacy check is made against a DC set by the dungeon master. The target’s general attitude toward you (friendly or unfriendly, peaceful or hostile) and other conditional modifiers (such as what you might be seeking to accomplish or what you’re asking for) might apply to the DC. Diplomacy is usually used in a skill challenge that requires a number of successes, but the DM might call for a Diplomacy check in other situations.