While technically not a keyword ability or a keyword mechanic, ‘target’ is definitely a key word in Magic, and one that can confuse players. It is important to know which spells and abilities target because of the interactions with actual keyword mechanics like protection, shroud and hexproof, and for other rules issues like determining if a spell is countered on resolution.
In most cases, only those spells and abilities that use the word ‘target’ actually do target. When reading a Magic card, the word ‘target’ should jump out at you as if it were highlighted because of its importance. Spells like Day of Judgment or Rolling Temblor do not use the word target, and therefore do not target at all. They will affect creatures with protection, shroud or hexproof.
Similarly, it is important to make note of what is specifically targeted. Tribute to Hunger does target, but it targets a player. The creature that is sacrificed is not targeted.
Sometimes how a target is mentioned in a card’s text can seem a bit odd, and not all targets of a spell are listed at once. For example, Fulgent Distraction states that you ‘choose two target creatures.’ These targets are still chosen when the spell is played–it is worded this way as it makes it clearer that two things are happening to these two creatures when the spell resolves. Similarly, Into the Maw of Hell has two targets: one that is mentioned at the beginning of its text (the land) and another that is referred to at the end (the creature). There is not just one way that the word ‘target’ is used.
Finally, it is possible for a spell or ability to target and not have the word ‘target’ appear on the card. Keyword abilities that work in complex ways have their rules spelled out in the comprehensive rules. In most cases, these keywords also have reminder text that uses the word target. However, at least one keyword–‘Enchant’–does target and does not have reminder text. Aura enchantments do target when played, although that word does not appear on all auras.