Macros are basically 'super buttons' that you can create in order to use multiple abilities and spells from the same button. This means even if you have 15 different abilities, you can key them all to 3-5 buttons. With macros you can do things like:
1) have all your buff spells on one button
2) 'hide' abilities that you don't use very often so they only show when you need them
3) 'group' similar abilities so that, for example, all your commonly used melee skills are on one button
4) maximize your rotation so that you're always one step ahead of the GCD
Macros are great, but there are a few limitations:
1) complicated! -learning macros takes a good chunk of time. It's only something I've been able to do since my work gives me some 'free time' to cruise the UI/macro forum.
2) software limited -While very powerful, there is potential to abuse macros so that, for example, you could hit one button and your character would kill every mob in range (eventually). This potential for abuse has cause blizzard to put some powerful limitations on macros. More about this later.
3) the One Button rule -each macro button can use only one ability each time you press the button. You can code this so that it will use a different ability each and every time you press it (and on a different target!), but you can't press the button once and, for example, have it cast all 5 self buffs on you.
I don't even know where to start?!
To get started with macros, it's very simple. Just log on wow gold and type /m. This brings up your macro box, where you just have to hit 'new' and the game will prompt for a name and an icon. I recommend that you leave the icon as a question mark. (?) The question mark means the game will automatically pick up the picture of the ability that the macro will use.
My First Macro
For your first macro lets just replace a normal ability you use on your bar in combat. I'm going to use paladin abilities here, but you can replace them with whatever ability you like.
So to make my first macro, I went to the /m menu in game, picked a new macro, and gave it the question mark icon. Then I click on the icon, click on the empty box, and type:
/cast
This is the basic core of every macro. It's basically like the verb in a sentence. You can use different 'verbs' (called slash commands) but this is probably the most popular. Since I want this macro to cast Hammer of the Righteous, I simple type that in after the /cast.
/cast Hammer of the Righteous
And pow! I'm done! If you drag this onto your bar and hit it a million times, you basically have the paladin protection spec in a nutshell (lol). However this doesn't really give you any extra functionality over a normal keybind. The next thing I'm going to do is make the macro heal a friendly if I have a friendly targeted, or if it's an enemy it will use SotR (shield of the righteous). To do this you add a conditional. This says 'only cast this spell if 'X' is true.' It looks like this (with a breakdown of the macro underneath):
/cast [help] Word of Glory; [harm] Shield of the Righteous
\__/ \____/\__________/\/ \____/\__________________/
verb-conditional---|---separator|----------------|
-------------------ability---------conditional----Ability
What this macro will do is check the status of your current target. A 'friendly'(help) value will cause my paladin to heal the target. An 'enemy'(harm) value will trigger the attack ability. If you can grasp macros thus far, you've already learned enough to make macros useful for you. Putting this on your bar will basically cut the number of keybinds you need by 1.