To activate a Spell/Trap Card is to have it start a Chain Link by playing it to the field Face-up (either by play it from your hand face-up, or to have the card face-down on the field and Flip it face-up).
To activate an Effect is to have the effect start a Chain Link. This goes for most of Effect Monsters and some Spell and Trap Cards. 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/Trap Card or Continuous Effect Monsters never are 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 "Black Star Dragon" and "Necromancer of Dark Magic" - 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.