As posted in the Player's Handbook...
By choosing to delay, you take no action and then act normally on whatever initiative count you decide to act. When you delay, you voluntarily reduce your own initiative result for the rest of the combat.
You can ready a standard action, a move action, or a free action. Your initiative result changes. For the rest of the encounter, your initiative result is the count on which you took the readied action, and you act immediately ahead of the character whose action triggered your readied action.
Delaying your action will alter your initiative in combat, acting as normal. Readying an action however, will not alter your initiative, simply acting as a change for that specific round. Remember though that if the predetermined conditions do not take place than your action for that round is lost.