Here is all the info that I could find on Focus, its not mine so I take no credit for it.

Saving a target for later action

The /focus command allows you to save a target to come back to later. For example, say your raid leader assigns you a target to sheep. First, select that mob, and type /focus. Now you can use a macro like the following to cast sheep on your focus.
/target focus
/cast Polymorph
/targetlasttarget
Using Focus

Focus is a unit ID like target, player, or raidpet1target (See http://www.wowwiki.com/UnitId). It allows you to reference a mob, player, or NPC you specify. The simplest usage of focus is with key bindings. There are two focus-related functions in the bindings menu: Focus Target, and Target Focus. Focus Target sets your focus to whatever you are currently targeting (it will clear your focus if you have nothing targeted). Once you have a focus set, you can use it as a unit ID for any other command. Target Focus will, as you might guess, target the entity you have focused. However, these bindings don't really take full advantage of focus. In order to get the most bang for your buck, you will need to use macros with macro options.

One of the most common uses is to set a crowd control target. A mage can select a mob to sheep and set it as their focus. Now they can change targets for DPS and use the following macro whenever they need to re-sheep.
/cast [target=focus] PolymorphOr maybe a healer could set their focus to the main tank. With an addon like FocusFrame (http://wow.curse.com/downloads/details/5681/), they would then have a frame devoted to their main tank that they could easily use for healing.

In addition to the key bindings, there are also the /focus and /clearfocus slash commands. Without any parameters, /focus works exactly like the key binding, setting your current target as your focus. You can also specify any valid unit ID (see Targeting above) or name as a parameter to /focus:
/focus party3targetHere is an example of more advanced focusing:
/focus [target=focus, noharm] [target=focus, dead] [modifier]
/stopmacro [target=focus, noexists]
/cast [target=focus] PolymorphThe first line sets your focus to your current target (or clears your focus if you don't have a target) in one of the following situations:
You don't have a harmful focus (either it's friendly or doesn't exist)
Your current focus is dead
You are holding down a modifier key (in case you want to change your focus after you already have a valid one)
The second line keeps the macro from proceeding if you don't have a focus. Finally, it casts Polymorph on your focus. This gives you a one-button solution for your crowd control with focus. You may notice that we could have used an exists condition in the /cast command instead of a separate /stopmacro command. However, /stopmacro affords us a bit more flexibility by stoping any other commands we may add to the macro later (like a warning in /p).

It's possible to swap your target and your focus, giving you in effect two targets you can toggle between:
/cleartarget [target=target, dead]
/clearfocus [target=focus, dead]
/target focus
/cleartarget [target=focus, noexists]
/targetlasttarget
/focus target
/targetlasttargetThe first two lines clear the target and/or focus if they are dead (if you really want to keep track of multiple dead targets, e.g. to resurrect or loot them, then delete those lines). The fourth is needed because /target focus doesn't clear your target if you have no current focus (without it, the fifth line would then retrieve your previous target).