A playing card is easily recognized by the two symbols on the face of the card in the upper-left and lower-right corners — a ‘suit’ and an ‘identifier’.
A deck of 52 cards is divided evenly into four ‘suits’ — Clubs, Diamonds, Hearts, and Spades. This allows you to have a flush, cards all of the same suit (Cribbage, Poker). This also allows trick-taking games that use a trump suit and that require playing a card ‘with the suit that was lead’ (Bridge, Euchre, Five-Hundred, Pinochle, Whist). This also allows games where you have to play a card with the suit that was last played (Crazy Eights).
Each of the four suits has 13 cards that are marked with the same 13 ‘identifiers’ or ids — 2, 3, 4, 5, 6, 7, 8, 9, 10, J, Q, K, A. So for each id, there are exactly four cards, one for each suit. This allows you to have two cards that match (Concentration, Crazy Eights, Old Maid, War), two-or-more-of-a-kind (Cribbage, Go Fish, Poker), three-or-more-of-a-kind (Canasta, Gin Rummy, Rummy), and special combinations of matching cards such as a full-house, a pair plus three-of-a-kind (Poker).
Four of the ids are ‘letters’ which represent special cards — three face-cards (‘J’ = Jack, ‘Q’ = Queen, and ‘K’ = King) and an Ace (‘A’ = Ace). This allows you to have special scoring for these special cards (Hearts, Michigan, Pinochle, Tripoli). Each face-card has a double picture of the King, Queen, or Jack so that there is always a right-side-up picture on the upper half of the card. The Ace has a suit at the center.
Nine of the ids are ‘numbers’. This allows you to add the number value of the cards. Face-cards and aces can be included by assigning a value such as 10 for face-cards and 1 or 11 for Aces (Blackjack, Cribbage). In addition to the two suit symbols, a number of suits symbols equal to the id number are printed on each number card.
The suits have two ‘colors’ that divide a deck of cards evenly in two — Black (Clubs, Spades) and Red (Diamonds, Hearts). This allows you to link cards from two suits with the same color such as using the Jacks of one color as high trump cards (Euchre, Five Hundred).
Each of the 13 ids is part of a ‘sequence’ of ids — usually from ‘A’ (high) to ‘2’ (low). This allows you to have a straight, five cards in sequence (Poker), or a run, three or more cards in sequence (Cribbage). If these cards also have the same suit, you can have a straight-flush (Poker) or a single-suit run (Gin Rummy, Rummy). You can also lay down cards of a given suit in sequence (Fan Tan, Michigan).
The ids are also called ‘ranks’ because each id has a rank when compared to any other id — again the highest id is often ‘A’ and the lowest ‘2’. This allows you to have the high-card take a trick (Bridge, Euchre, Five Hundred, Hearts, Pinochle, Whist). This also allows you to have comparison games where a high-rank card takes a low-rank card, and cards of equal rank lead to war (War), and a high-rank pair beats a low-rank pair (Poker).
In addition to the regular cards, a deck of cards may also contain two Jokers. These cards have the word ‘Joker’ in the upper-left and lower-right corners. This allows you to add a wild card or two to games (Canasta, Five Hundred, Poker).
All cards in a deck of cards have identical backs containing anything from a solid color to a complex design. This allows you to see the value of your own cards but not your opponent’s cards (most games). This also allows you to play games in which you try to remember the location of face-down cards (Concentration). And this allows you to play games where a participant can look at cards, and then lay them face-down so that others cannot see them (Texas Holdum’ Poker).
And the suits and identifiers and face-card pictures and backs are all printed on card stock with a plastic coating or on plastic. This makes the cards durable and allows you to play plenty of card games, alone or with friends (Blackjack, Bridge, Canasta, Concentration, Crazy Eights, Cribbage, Euchre, Fan Tan, Five-Hundred, Gin Rummy, Go Fish, Hearts, Michigan, Old Maid, Pinochle, Poker, Tripoli, War, Whist).