If you're plugged into the priest scene, you're probably aware of a talent-based mechanic in the priest holy tree known as Chakra. The purpose of Chakra is to allow the holy priest to "switch modes" between single-target and area of effect (AoE) healing, getting upgraded tools for each role when in the appropriate "stance."
The Chakra family of talents is not available to discipline priests, as it resides too deeply down the holy tree and requires an important spell that you only receive if you select holy as your specialization.
However, the way that discipline priest healing has shaken out this expansion, discipline priests work in very much the same way...they just don't activate Chakra before switching modes.
This is either "traditional discipline" or else "new wave discipline" depending on whether you like the Evangelism/Archangel/Atonement mechanics and, if so, how you use them. I'll have a future article discussing "new wave discipline" and clarifying the best way to look at it, in my experience. For now, though, let's quickly summarize each.
Traditional discipline works this way when you're healing someone (usually a tank) through continued and reasonably intense damage: you put a shield up. You get a Prayer of Mending on the target. You begin casting Heal (if the shield hasn't been broken and the damage isn't expected to be massively spiky) or Greater Heal otherwise. The reason you immediately start casting those spells is because they will reduce the duration of the Weakened Soul debuff, allowing you to get a new shield up as soon as possible and have a further benefit of stacking up some Divine Aegis shielding when and if you critically heal. As soon as the shield has broken and the tank is actually taking some damage, use Penance on cooldown, refreshing Power Word: Shield and using Prayer of Mending whenever they're available. The rest of the time, you use Greater Heal and reserve Flash Heal for emergencies when the target's health is dropping toward death quickly. If you and the target are both hurt, Binding Heal is reasonably efficient.
If the target (like a DPS or fellow healer) is not taking big, consistent damage, you simply spot heal with Penance or Greater Heal (using Binding Heal if you are also damaged).
Healing is a dynamic, situational endeavor that doesn't lend itself to spell rotations, but that's the general work flow. You'll adapt it based on how the fight is going.
The Atonement style ("new wave!") is actually not hugely different in execution, but does have the added novelty of weaving in some Holy Fire and Smite. This mechanic also gives you an extra button to press in Archangel, a healing buff on a 30 second cooldown.
So that's single-target healing in a nutshell. Let's see how area of effect healing is done in discipline and how it differs in spell usage.
Area of Effect Healing
Using the term "area of effect" is a bit of a misnomer, because the way discipline priests heal a number of people really isn't based on a specific area (outside of the range of their spells, of course), but so many mass heal spells are area-based, so it makes a good catch-all term. This is generally for periods when raiders stack up for a big "burn" phase which are extremely common in Firelands (Beth'tilac phase 2, Lord Rhyolith phase 2, Alysrazor phase 4, Majordomo Staghelm scorpion phases, Ragnaros phase 2).
Here's how you do it with discipline: Select a group in your raid and cast Prayer of Healing on that group. Select a different group and cast Prayer of Healing on it. Select a third group (if you're a ten man raider, this will be the original group) and cast Prayer of Healing on it. Etcetera.
(The reason you want to alternate/switch groups each time is not just to balance out healing but also to avoid clipping the heal-over-time [HoT] that this helpful glyph provides when you cast Prayer of Healing.)
To be sure, there are some nuances you can take advantage of...for example, casting a Power Word: Shield on someone (like a tank) between Prayers of Healing in order to haste each Prayer with Borrowed Time. If you and someone else are going down in health faster than others for some reason, you can resort to Binding Heal. Once and only once per fight you can toss out a Divine Hymn, which will also boost your fellow healers' efforts. You also want to keep Prayer of Mending out on cooldown.
By and large, though, you ride Prayer of Healing to victory. That's discipline AoE healing: spam Prayer of Healing. There is no toolbox, just one (pretty powerful) tool.
Discipline priests, then, have two pretty discrete "modes" that they switch between. In one they use a pretty diverse toolbox (there are more spells that can be used situationally that I didn't mention, like Renew) whereas in the other, the toolbox nearly collapses down to one spell, like a star collapsing into a neutron star before exploding in supernova.
Sometimes those modes line up nicely with phases, but sometimes they don't. Therefore, keeping a bit of the holy Chakra mindset is good even in discipline; the way in which you heal and the spells you use change and the difference is very stark. You just won't be pressing Chakra when you want to switch modes, but the mindset should be the same.
At any given moment, be ready to let your toolbox collapse into super-dense Prayer of Healing spam and let loose an AoE healing supernova.