Focus was added so people could use it to keep track of a mob or player. It is a target frame. You can get an addon that shows it. I use FocusFrame from curse-gaming.com

I don't like to change targets, almost ever. I use macros to try to keep me from directly targeting or having to change targets. I focus my main. Meaning, on my alts, I use...
/focus [target=party1, nodead] [target=party2, nodead] [target=party3, nodead]
I don't go past 3, cause that pretty much is a wipe. Also, I can still target when my main is dead and the macros still work, so I almost never change my focus. So that gets my alts a person to cast on and uses focus-target to get a mob to kill.

For macros that use focus, I use
/cast [target=focus-target] DPSSpell for DPS
/cast [target=focus-target-target] HealSpell for healing

The alts never have to target anyone, they just cast on whoever the main is targeting. They don't have to know who has aggro, they just heal whoever the mob is hurting. That is mostly cause I don't have a designated tank on my hunters or my shammies. I hope they equally get aggro so it spreads the damage out. So I don't know when anyone might steal aggro, so I make a macro that doesn't care. It just heals whoever is getting hurt.

Now to your question. You can use castsequence to cast a sequence of spells. It has a lot of features, so you can either look them up or just ask here and most people can help describe them. One is reset, one is options and one is targeting.
First, reset. You can force a reset at a specific time by putting in dummy commands so the sequence ends when you want.
/castsequence reset=16 Power Word:Shield, Flash Heal, , , , , , , , , , , , , , , , , ,
If you spam that, you get a shield and then a heal and then nothing until 16 secs (picked for 15sec debuff of PW:S)
Can skip options, but some are combat, dead, exists, party/raid, channeling, flyable, mounted, equipped, etc.
The targeting helps like I described above.
For heal:
/castsequence [target=focus-target-target] reset=target/combat/16 Power Word:Shield, Flash Heal, Flash Heal, Flash Heal, Flash Heal, Flash Heal, , , ,
For DPS:
/castsequence [target=focus-target] reset=target/combat/20 Siphon Life, Curse of Agony, Corruption, Unstable Affliction, Immolate, Seed of Corruption, , , , , , ,
The target is so they start over on a new target, the 20 is so you can spam it in case you get excited and it doesn't try to start over, wasting DOTs.


Hope that helps.