To activate an effect is to have the effect start a Chain Link. This goes for Effect Monsters, Spell Cards, and Trap Cards. Continuous Effects never activate, since they never start Chain Links.

To activate a Spell or Trap Card is to have it start a Chain Link by playing it to the field face-up. That is, to activate a Spell or Trap Card is to play it from your hand face-up, or to have the card face-down on the field and flip it face-up. Monster Cards cannot be activated.

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.

See Also: Chains, Activation, and Resolution.