It's the question that always comes up without fail. Whether you are a player or a judge, there will ALWAYS be a question about a Card targeting or not. And it's not because the mechanic itself is hard to grasp, it's because everyone has a different understanding of what "targeting" means. There aren't that many Cards that involve being targeted, but the mechanic becomes very important when it defines whether you must or must not tell your opponent which Cards you are going to affect.
First, let's explain what constitutes targeting: If an Effect targets, then you are required to select a certain number of Cards upon the Effect's activation. Once again, much like the definition of priority, that mere line says all that you need to know. And of course, again, this line says a lot more than it looks like.
The first part tells us that you are required to choose monsters. It may seem evident, since you need to "target" those monsters. Do note, however, that this becomes a requirement for effects that do target. For example, you do not need to make any choices while activating Mirror Force or Card Destruction, as they affect "all" Cards of a certain group. It doesn't matter if your opponent controls 1, 3, or 5 Attack Position monsters, or if a player has 5, 10, or 20 Cards in his/her Hand, the Card can still resolve perfectly without you having to pick any peculiar Cards. This tells us that Card Destruction and Mirror Force do not target. On the other hand, a Card like Book of Moon only affects 1 monster, and that monster can be almost any face-up monster on the Field. Unless you choose "that" monster, Book of Moon cannot decide by itself which monster should be flipped face-down. This tells us that Book of Moon target.
The second part tells us that the targets must be chosen at activation. That is to say, any monsters that you address before or after the activation are not considered to be targets. For example, paying a cost happens slightly before activating the effect, so in order to tribute Sangan for Crush Card Virus, you do not need to target it. On the opposite end, certain Cards will decide which Cards will be affected at their resolution. Since the resolution comes after the activation, these Cards do not target. For example, Saber Slash counts the number of X-Saber Monsters at its resolution, and only then, one knows how many Cards to destroy. Since the Cards to be destroyed are chosen at Saber Slash's resolution, it does not target.
While the definition tells us that Cards target at activation, some Cards also target the monster continuously. This happens mostly when the "targetter" is related to its "target", usually by a doom clause. For example, Call of the Haunted targets a monster in the Graveyard upon its activation. Additionally, it targets the monster continuously once it has resolved, in order to maintain its doom clause (if Call is Removed from the Field, the monster is destroyed, and if the monster is destroyed, so is Call).
In most cases, the text of a Card won't tell you that it targets. The following are some rules to help you determine whether a Card targets or not, along with the first definition at the beginning of the article:
1) Costs do not target, no matter what the cost does. For example, one of Dark Armed Dragon's rulings reads:
•""Dark Armed Dragon's" effect that destroys 1 card is an Ignition Effect. Removing 1 DARK monster from play is a cost."
So, Removing a DARK monster from Play does not target that DARK monster.
The opposite is also valid:
•""Offering to the Snake Deity" targets 1 Reptile-Type monster you control and 2 cards your opponent controls."
This tells us that destroying your Reptile-type monster is not a cost.
2) If the Card can "find" the other Cards it will affect by itself, without you having to move a finger, then it does not target. For example, Card Trooper can only increase its own ATK by its effect, so it does not require you to choose Card Trooper as a target. Therefore, this effect does not target. On the other hand, Blackwnig - Sirocco the Dawn can increase its own ATK, but it can also increase the ATK of other monsters. Since you are forced to choose a monster whose ATK to boost, Sirocco's effect targets.
3) An effect cannot be both, targeting and non-targeting, for the same action. For example, Smashing Ground does not target due to rule #2 (it searches the monster with the highest DEF by itself). In the case in which two or more monsters have the same DEF, the decision is made by the player, since Smashing Ground can only destroy 1 monster. However, we ruled that Smashing Ground does not target, so even in the case in which you must choose one of these monsters, this decision is made at Smashing Ground's resolution. Simply put, Smashing Ground NEVER targets.
4) If a Card affects "all" Cards of a certain range, it does not target. This one is very similar to rule #2. For example, Judgment Dragon destroys "all other Cards". You do not need to target "all other Cards", as they will be every other Card that isn't this particular Judgment Dragon. At the same time, you do not need to target Judgment Dragon either, as it is not a Card that will be affected. And so, Judgment Dragon does not target.
5) If the effect affects a certain number of Cards, and this number is decided at resolution, the effect does not target. For example, Elemental Hero Stratos' destruction effect counts the number of heroes you control during its resolution, and it also lets you manipulate the number of Cards to destroy. And so, this effect doesn't target. You can also use the Saber Slash example at the top.
6) All Equip Cards target, upon activation and continuously after they resolve. Take any Equip Card of your choice as an example Q_Q
7) Cards in the Hand, Deck, and Extra Deck cannot be targeted, ever. This implies that the only Cards that can be targeted are those on the Field, Graveyard, and Removed from Play Zone. So, for example, Reinforcement of the Army and Trap Dustshoot do not target.
8) Due to rule #7, if a Card can affect Cards in one or more of the zones that cannot be targeted, then it does not target, even if the other zone(s) can be targeted. For example, Gigaplant and Red-Eyes Darkness Metal Dragon can Special Summon monsters from the Hand or the Graveyard. Since Cards in the Hand cannot be targeted, neither the plant or the dragon target. In fact, you decide the location of the monster that you will Summon during the effect's resolution.
9) If an effect negates others by chaining to them, the negation effect does not target. For example, My Body as a Shield or Gladiator Beast Heraklinos will always negate the Card directly behind them in the chain, so it doesn't target. Be careful not to confuse this with negation Cards that do not use the chain, such as Skill Drain or the Imprisoning Mirrors.
10) From rule #9, we have that "most" Counter Trap Cards do not target, since most of them are devoted to negating activations. The only three that target are Intercept (targets a Summoned monster), Gemini Counter (targets a Gemini), and Negate Attack (targets the attacking monster).
11) You need to be a little careful with this one. Most Cards that involve one or both battling monster(s) do not target. For example, D.D. Warrior Lady, Legendary Jujitsu Master, and Honest do not target. However, there are some rare examples in which battling monsters are targeted, such as Dimensional Prison.
As we have seen so many times in this article, targeting occurs at activation. This means that, in order to actually place your effect in the chain, you MUST declare all of your targets before either player can respond. So, for example, if you activate Icarus Attack, aside from paying its cost, you must tell your opponent which two Cards are on the Field before priority is passed to him/her. Once you choose your targets, you cannot ever modify them, so if I target my opponent's Gemini Monster with Icarus Attack, and s/he responds by tributing it with Gemini Spark, I cannot change the targets of Icarus Attack to a different Card. Similarly, Cards that do not target do not declare any targets, so if you activate Return from the Different Dimension while having 8 Removed from Play monster, and your opponent uses Burial from a Different Dimension on 3 of them, you can still Special Summon the other 5. You never need to declare that you were going to Summon "Jinzo", so if your opponent returns it to the Graveyard, Return can still Special Summon other monsters.
As a final note, please, please, PLEASE do not use the word "target" for Cards that do not target. It can be somewhat hard to tell them apart at the beginning, but some people are doing out of pure vice. "How many Tomato targets do you run?", "Are ten targets for Allure enough?", and many more. If you are meditating your Deck with yourself, that's ok, but mentioning it in front of other players who aren't familiar with the mechanic leads them to think that Cards like Mystic Tomato target.
If you have any questions, feel free to drop me an e-mail at ness00[at]gmail[dot]com.