Discard 2 cards.
All other copies of this in play are discarded (belonging to any player). At the start of your next turn, + 2 Cards.
While this is in play, when each other player plays an Action or a Treasure, they discard a card.
Discard 2 Cards when you play this, and at the start of your next turn draw 2 Cards. If this card is removed from play before your next turn (by another Siege card, for example), you will not get to draw the 2 Cards.
When Siege is played, all other copies of it are discarded from play, including your own. When played multiple times by effects like the Throne Room, there isn't another copy in play, so the card is not removed. However, the persistent effect on other players only counts copies in play, so a Siege played via King's Court will still only force other players to discard one card per card played, not 3.
While this is in play, any time another player plays any card they must immediately discard a card, before their card has any effects. The owner of the Siege is not affected by this.
One card must be discarded for each card played, as they are played. If player doesn't have any more cards to discard, they can still play the last card from their hand.
For example, a player who has 5 Coppers in hand could play a Copper, discard a Copper; play another Copper, discard another Copper; and finally play their last Copper, resulting in a total of 3 Coins for their Buy phase, instead of 5.
Revealing a Reaction would force a discard, just as playing an Action on the table.
A card which protects against attacks, like Moat, can be revealed when Siege is first played and will provide protection for as long as the Siege is in play. Revealing the Moat on later turns is too late. Use player tokens, as described in the Duration-Attack rules to remember that a particular player is immune to a Siege. This immunity includes not having to discard any Siege the defender has in play. In this way multiple Siege cards can be in play at the same time.
If there are multiple copies of Siege in play, the discard effect stacks. So if there are two Sieges in play, every time a card is played, two cards must be discarded (not for everyone though, keep in mind players are immune to their own Siege).
If it isn't removed prematurely, Siege will remain in play until the Clean-up phase of your next turn.