Smite is a community build, not one of the game's three priest specs. It puts Discipline and Holy talents behind Holy damage: 
Holy Fire burns on the boss so that Power in Light makes every 
Smite hit harder, 
Shadow Word: Pain and 
Devouring Plague tick underneath, and Smite fills every other cast. Mana sets the pace on long fights, so you switch to a cheaper rank of Smite whenever you are spending faster than the fight allows. Its score on a one-minute single-boss fight is 633.5 DPS, 23rd of 25 damage specs.
Strengths and weaknesses
Strengths
- No hit to chase for your Holy spells. On the Classic spell table our simulator uses, a raid boss resists 17 % of your spells and hit can remove all but 1 % of that.
Holy Precision gives your Holy spells 18 %, so Smite and Holy Fire miss just the 1 % that always misses. Gear hit only helps your two Shadow spells. - The same damage at any fight length. On one boss your number barely moves: 633.5 DPS on one minute, 597.6 DPS on three. Most builds have no burst cooldown to wait for.
- Shadow Word: Pain on every boss. Against three bosses it goes on all of them, does 41.2 % of your damage, and lifts your score to 679.5 DPS.
Weaknesses
- Near the bottom of the table. On a one-minute single-boss fight Smite reaches 58.1 % of the best damage spec's number. Smite and Holy Fire hit one target, so the gap is wider on three bosses: 48.9 %.
- Mana. Over three minutes on three bosses a
Major Mana Potion is worth 13.1 % of your damage, and on one boss for three minutes many of your Smites are a cheap rank. - One rule nobody has tested. That cheap rank is rank 4, 17.8 % of your damage on a three-minute single-boss fight. Our simulator gives it full spell damage. Blizzard's notes of 1 October say spells far below your level get less from spell damage, without saying how much, so the three-minute number may be high.
On every fight
Smite's rank barely moves: 23rd on one boss for three minutes, 24th against three. Three bosses raise your score, because Shadow Word: Pain goes on each of them, but the specs with area spells gain more. Choosing a fight in this table sets it for the whole guide.
Races
On a one-minute single-boss fight the best Alliance race is Night Elf. Gnomes lead every fight except one boss for one minute. Expansive Mind adds 5 % maximum mana, and 
Eureka! makes your next three direct spells cost 10 % less and hit 10 % harder every two minutes. On the one-minute fight the Night Elf's Elune's Light, 10 % crit for 15 seconds at the pull, puts it level with the Gnome.
On the Horde side, Undead lead the two-minute fight and every three-boss fight. 
Dark Sacrifice, a spell only Undead priests learn, turns 1,600 health into 1,600 mana plus your Spirit over 15 seconds, and 
Touch of the Grave drains health from the boss on your spell hits. Trolls lead one boss for one minute with 
Berserking, 10 % casting speed for 10 seconds, and are level with the Undead on three. The gaps between races are small, as the table shows.
What a Priest brings to the raid
Every priest brings 
Prayer of Fortitude, 
Prayer of Spirit and 
Prayer of Shadow Protection. Divine Spirit is baseline in Forever, so Prayer of Spirit costs you no talent points. Below is each buff's worth on this fight to the specs it helps. Shadow Protection scores nothing here, because the simulated boss only hits physically.
Prayer of Fortitude+0.4 % to 3 of 25most: Mutilate Rogue +0.5 %
Prayer of Spirit+0.3 % to 3 of 25most: Elemental Shaman +0.4 %
Changes
Each point is one simulator or game-data update and where this build's number landed after it. Updates explains each one.