Activate

To activate a Spell/Trap Card is to have it start a Chain Link by playing it on the field Face-up, either from your hand or fliping it after setting. Trap Cards and Quick-Play Spell Cards can't be activated on the same turn they were Set.

To activate an effect is to have the effect start a Chain Link. This goes for most of Effect Monsters and also for Spell/Trap Cards which have Ignition-like, Trigger-like or Quick-like Effects. Effects can be activated not just when the card is face-up on the field, but also in the hand, Graveyard, Banished Zone, Main and Extra Deck, depending on what the card says. Continuous Effects of any Continuous Spell/Continuous Trap Card or Effect Monsters never activated; and never start any Chain Link.

Note that "activating a card" is different from "activating the effect of a card". For example, some effects - like "Magic Jammer" and "Skilled Dark Magician" - only apply to when a Spell Card is activated, but do not apply when the "effect" of a Spell Card is activated.