Duration is a card type that was introduced in Seaside. Duration cards are Action cards which usually have an effect on the turn after they are played. A Duration is not discarded from play until the last turn on which it does something.
On the turn on which they are played, Duration cards typically have weak effects (relative to their cost), or even detrimental effects; their strength comes in part from the fact that their effects on the next turn do not consume an Action to play or a card slot in your hand. For example, Caravan, on the turn it is played, is a cantrip with no effect; on the next turn, however, it gives you a sixth card in your hand for free—thus playing a Caravan on one turn has the same effect as playing a Laboratory on the next turn.
Although Duration cards' next-turn effects are typically quite strong, they cost less than and are usually considered weaker than cards that have those same effects immediately—for instance, Caravan costswhile Laboratory costs . This is in part because it's better to receive benefits earlier—playing a Caravan on the last turn of the game gives no benefit while Laboratory does—and in part because the fact that Durations stay in play for two turns is a disadvantage, since it means that they can actually be played less frequently. For instance, it's possible to draw an extra card every turn by having a single Laboratory and playing it every turn; in order to play a Caravan every turn, you have to have two copies of the card.
The durations are
A Haven played with no card in hand to set aside or a Tactician played with no cards in hand to discard will be cleaned up (discarded from play) at the end of the turn when they are played; in all other cases, the duration will stay in play until the end of the turn after it was first played.
When you Throne Room or King's Court a Duration, the Throne Room / King's Court also stays in play until the Duration is cleaned up, to remind you that the Duration's next-turn effect needs to happen twice or thrice. When you Procession a Duration, the Duration gets trashed, but it still has its next-turn effects, and the Procession remains in play until the time at which the Duration ordinarily would have been cleaned up if it remained in play. None of this applies to Tactician; since its main effect can't really be "throned", the accompanying card is discarded at the end of the turn, though Tactician is not.
Although the Seaside rules do not mention this explicitly, a Duration card still in play when the game ends is returned to its owner's deck before scoring; this can matter for alt-VP cards such as Gardens.