The pack of cards you deal, and possibly later draw, from.
Technically the Stack is the pile of cards which you later draw from. The deck becomes the stack after you deal your hand, in most games.
The cards you hold in your hand, concealed from the other players.
Cards are dealt out one at a time. The dealer starts dealing from the player to their left, clockwise, and deals the last card to themselves (assuming each player gets the same number of cards). It is usually considered bad manners to look at your hand before the dealer has looked at theirs, and some games (such as strict /Canasta) may penalise you for it.
While other games, such as blackjack, require it to function...
Cards form an ordered set for the purposes of comparison. Usually, the order of lowest to highest value is 2,3,4,5,6,7,8,9,Jack,Queen,King; the Ace comes before the 2 if it is low, or after the King if it is high. High is more usual; some games permit both.
If a game uses the notion of a trump suit, this means that cards of that suit are treated differently to other cards; usually, they are ranked higher than the other cards. So the ordered set becomes ...nontrump 8,9,Jack,Queen,King,Ace, trump 2,3,4,5,...