Macros are an important tool in the World of Warcraft game, but one which a relative minority of players actually understand and make use of. They are extremely useful in nearly any situation, but in endgame raiding and PvP at all levels, they are close to indispensable if you want your character to achieve the peak of their role.
A macro is a short "program" that you write in the macro interface and assign to a button, when then becomes part of your character's macros. You can make character-specific macros or write macros which are available to all characters on your account. Generally speaking, macros which cast spells and/or use other class abilities and attacks should be set up as character specific macros, since they will not benefit characters who lack the spells and abilities used by the macro.
Clicking on the button assigned to the macro (which you can label and drag out to your character's actionbars as usual) will trigger it. Whatever actions you wrote into the macro will be executed automatically, assuming that the actions can actually be taken, and that the macro has been correctly written. There are limits to how many different actions can be written into a specific macro, of course, to prevent you from automating your character's entire battle, but these small programs are still highly useful.
Macros are useful precisely because they can cast a series of spells or abilities faster and more certainly than human reflexes can. Pressing the bound key for a macro causes your character to carry out the actions programmed into it, regardless of whether you are confused or distracted by a struggling mass of PvP combatants surging over your screen, startled by the damage spike caused by a nearby boss, or otherwise not at your peak.
Clicking on buttons with your mouse cursor is the slowest and most ineffectual method of triggering spells and abilities. By the time you click, your character is likely to be dead – or the tank they are supposed to be healing is stretched on the floor of some dreadful, ancient temple with a wrathful boss monster bearing down on the group, or something equally undesirable.
Using keybindings and pressing hotkeys on your keyboard to cast spells is infinitely more effective – and employing macros is a step up again from that. Your character's success as an endgame raider, PvP fighter, or arena team member will hinge on the quality of your keybindings and macros – and good macros can make your World of Warcraft play experience a lot smoother before the endgame, too.
The innovative guide to macros and keybindings, IWinButtons, will give you all the information you need to make powerful macros and use them effectively, with information for all classes, talent specs, and potential roles, both in PvE and PvP action.