One-shot effects perform some action and are done; for example, they could cause damage to be dealt, permanents to be destroyed, move cards from one zone to another, or create tokens. One-shot effects are only generated by resolving spells and abilities; static abilities always result in continuous or replacement effects.
Example. Lightning Bolt generates a one-shot effect as it resolves; in this case, it causes the card on the stack to deal some damage. Other examples of one-shot effects are Llanowar Elves, Terror, Unsummon, Nekraatal, Dragon Blood, Young Pyromancer.
However, one-shot effects can sometimes affect the game at later times. They do so by creating delayed triggered abilities that will go off at the appropriate time.