One-shot effect

One-shot effects are effects which do not last for an extended period of time.

Examples
Some examples of one-shot effects include the following:
 * Direct damage or burn, characterized by the card Lightning Bolt
 * Lifegain, characterized by Healing Salve
 * Land destruction, as represented by cards such as Sinkhole
 * Fast mana sources, such as Dark Ritual or Seething Song
 * Creature removal in the forms of Terror and Swords to Plowshares

Cantrips
Some of these one-shot effects also "replace" themselves in the form of cantrips. Often, a card will have a slightly heightened mana cost in exchange for the phrase "Draw a card" in the spell's text box. This allows little to no loss, and even the possibility of gaining card advantage and tempo.

Slowtrips
Slowtrips are merely delayed cantrips that were introduced in Ice Age and Coldsnap. While they do not have the "immediate" effect of a cantrip, they still allow the card being played to replace itself. The text associated with a slowtrip is "Draw a card at the beginning of the next turn's upkeep."