The GCD, or global cooldown is the period after casting a spell where you cannot cast another spell. Most regular spells (PW:S, Renew, PoM etc.) are on the GCD. The GCD is 1.5 seconds (I think) baseline, reduced by haste - for example, when spamming renew, you can still only cast one every 1.x seconds (depending on haste) - that's the GCD working. You can only have one spell that triggers this GCD in a macro.
Most self-buff cooldowns do not trigger the GCD - Inner Focus, for example (do holy priests have IF? I'm going off of Cataclysm info here.) Thus, these can be macroed alongside GCD spells - for example, the following would cast IF whenever available and serve as a regular Greater Heal button otherwise.
/cast Inner Focus
/cast Greater Heal