Difference between Costs and Effects

Typically, Costs are determined at Activation, while Effects are determined during resolution of the card.

Example: Cost

Magical Stone Excavation: Discard 2 cards, then target 1 Spell Card in your Graveyard; add it to your hand.

Notice here, that the controller is required to Discard 2 cards in order to meet the Activation Requirement for the card in question. Then, during resolution, the Spell Card is added to the hand.

Example: Effect

Graceful Charity: Draw 3 cards, then discard 2 cards.

Now here, there is technically no cost for activating the card, since Drawing cards is not considered a cost. This also means that the 2-card discard is treated as an effect.