First off, what is a focus target and what exactly is the point of this?
When you select a unit (NPC or PC), it becomes your target. You can also set this unit as your "focus" and it will be 'held' as the focus target even if you de-select it or select a new target. You can now use the focus command in a macro, to cast a spell at that second unit that you are not even targeting. This greatly increases your fighting prowess, and you are now also able to get around one of the slopiest parts of fighting multiple players in PvP: targeting.
Now you should be salivating. Imagine shooting your DPS target, while Siliencing Shot/Scatter Shot are sent at the healer without ever switching targets. With some small adjustments (listed in the first step) you can also watch the HP/mana of your focus target as well as see the castbar. Obviously, this has PVE applications as well, with Misdirection, for example.
Before we start, I would like to mention that setting this all up will in no way impact your regular play. If you don't use it after set up, you will notice no difference and your regular hotkeys will still behave as normal.
Step #1: Set up your new hotkey
The focus command doesn't require a mod, as it is a part of the deafault Blizzard package, but you must assign it a hotkey. To do this, open your key bindings and scroll down to targeting functions and find "Focus Target" and assign a new hotkey. For our examples we will use "ALT+f". You, of course, can use whatever is most comfortable for you.
Now, be sure to apply the change and exit the key bindings. You can play around now by targeting units and pressing your hotkey (ALT+f).
* Selecting a target and pressing the hotkey will set a FOCUS TARGET
* Selecting nothing and pressing the hotkey will empty the FOCUS TARGET
You will notice a different style border surrounds the frame of a unit that you have marked as your FOCUS. If you have a unit frames add-on like Perl, you will be very pleased to see that a second target frame is displayed when you have your focus set. This allows you to watch two targets' health/mana as you fight both of them, as well as showing the casting bar for both players. If you don't have a unit frames add-on, and don't want to change the way your layout looks but you DO want to be able to watch your focus target and casting, then you can download a stand alone add-on such as FocusFrame.
Step #2: Make a macro that will shoot at the focus target
For our example we will use Silencing Shot, but you will want to change it to the ability of your choice.
To start simple, here is a macro that you could drag onto a button. Clicking the button would shoot Silencing Shot at your focus target:
/cast [target=focus] Silencing Shot;
This is fine, but it will require you to add a lot of hotkeys that you might not have room for.
A better idea is to create a new macro, give it the same icon as Silencing Shot and then drag it onto the bar and replace the old Silencing Shot button:
/cast [button:2,target=focus] Silencing Shot; Silencing Shot
This macro will cast Silencing Shot if you use your normal hotkey, but if you right click the icon (not a left click, like normal) it will cast Silencing Shot at your focus target.
This is a good method to start out on because your hotkeys will still behave as normal and your regular playstyle will be uneffected if you choose to not use focus targeting and casting.
The third option is to use a modifier (ALT, in our example) that would give your existing hotkey the ability to shoot at your focus:
/cast [modifier:Alt,target=focus] Silencing Shot; Silencing Shot
This way requires no clicking and is faster and better for PvP. However, it does use a lot more hotkeys and you must be sure you dont have your 'ALT+hotkey' already bound to another function or you may be activating not the macro, but what the Blizzard default hotkey is.
Now you can watch the HP/MANA and castbar of another unit, as well as fire any special ability at that unit. I have a lot of my shots set up this way, just in case the focus target is suddenly at low hit points, and I want to Multi-Shot it, etc. I would recommend you set up Silencing Shot, Scatter Shot or Intimidate at least.
Macros
Note: /castrandom was said to have a higher priority than /cast and an ability to get around the strict nature of a castsequence loop. I don't know for sure, but it doesn't hurt to have it this way.
/castsequence reset=2 Raptor Strike(Rank 9), Wing Clip
/castrandom Mongoose Bite(Rank 4)
/castrandom [target=pettarget,exists] Lightning Breath
/castrandom [nomodifier,target=pettarget,exists] Kill Command
Gets out your first Raptor Strike + Wing Clip, as well as Kill Command/Lightning Breath and Mongoose Bite if you dodge or the Raptor Strike crits.
/assist [help, combat]
/stopmacro [help]
/castsequence reset=3 Steady Shot, !Auto Shot
/castrandom [nomodifier,target=pettarget,exists] Kill Command
/castrandom [target=pettarget,exists] Lightning Breath
/script UIErrorsFrame:Clear()
Will assist your friendly target if they are in combat, stop if they are not, or go into your shot rotation if the target is hostile. It also spams Kill Command, but if you use a modifier (like ALT), it will bypass Kill Command, so that you dont hang up your castsequence if your pet is out of range.
Note that parts can be combined into all your spell macros for ease of targeting. Here is our second example from Step #1 with the added lines on top:
/assist [help, combat]
/stopmacro [help]
/cast [button:2,target=focus] Silencing Shot; Silencing Shot
So you get your assist on friendly if in combat, stop if not in combat, Silencing Shot if hostile, and Silencing Shot to your focus if the button is right clicked.
/cast Feed Pet
/use [pet:wind serpent]Homemade Mag'har Grainbread; Blackened Trout
Will feed the pet bread if it is a wind serpent, otherwise it will feed the fish that my cat loves.
/cast [button:2,target=focus] Misdirection; Misdirection
Will cast Misdirection on your target, or misdirect on your focus with a right click.
/castsequence reset=10 Trueshot Aura(Rank 4), Aspect of the Hawk(Rank 8), Track Hidden, Call Pet
Gets you set-up for arena battle.
My advice on trapping is to set your trap target to your focus, and DPS your main target, get Distracting Shot on a focus macro, and take it easy while you DPS and keep your trap victim following you.