Reshuffle
A reshuffle in Dominion happens at a specific time and has a definite meaning.
A reshuffle means that all the cards currently in a player's Discard pile are shuffled together and turned face down as a stack of cards that will now constitute the player's Deck. ("Deck" can be a confusing term but technically refers to the pile that a player draws from, at least during the game).
A reshuffle occurs if and only there are no cards in a player's Deck (i.e., the pile that is drawn from) and the player is required to pick up a card from his or her Deck, whether to draw it, examine it, discard it, or for any other reason. Reshuffles may happen during the drawing part of the Clean-up Phase.
Practical example #1: You play a Smithy and need to draw 3 cards. However, you only have 2 cards in your (draw) Deck (and some other cards in your discard pile). In this case, you would pick up the 2 cards in your deck, then reshuffle your discard, make it your new deck, then take the last card needed from the top of this new deck. You would not reshuffle first and then take the top 3 cards of the new deck.
Practical example #2: Witch is in the kingdom and your opponent has bought one. After your turn, you have 5 cards in hand and none in your draw deck, and several miscellaneous cards in your discard pile. One of the cards you have in hand is a Smithy. However, you cannot choose to reshuffle until your turn when you play Smithy and are required to draw cards.
In this (second) example, the timing of the reshuffle has real, practical consequences. If your opponent plays Witch, (barring other conditions) you will gain a Curse to your discard pile; if your discard has already been reshuffled (when it shouldn't have been) into your new deck, you won't have the chance of drawing this Curse from Smithy. That's a distinct advantage to you.
There are other cases where this rule can change the outcome of the game, and therefore it is always in effect, though in practice in friendly games it may be bent when there are no consequences.
