Cards are two-sided items, able to be hand-held, that include play-relevant info on at least one side. They are generally rectangular with a 3:2 ratio, angled horizontally.
When “a card” is mentioned in isolation, the type of card usually referred to is a card from a “standard” 52-card deck.
Cards are most well known for the injection of luck into games they are present in. Cards tend to arrive as part of a set of multiple cards that are mostly indistinguishable when placed in a specific manner. (Typically, this manner is that one side of the card is uniform and the other is variable, allowing the placement of a card “face down”; with the variable side hidden and the uniform side visible.) As such, players have a set of items that are different in gameplay, but appear identical until “revealed”, naturally creating an element of luck when the cards are shuffled.
Another useful quality of cards, most often used in Trading Card Games, is the aggregation of additional rules. As cards are two-sided and tend to be larger than other play pieces, cards are suited for the inclusion of additional game rules and attributes on each individual card. This assists players as the additional rules written on a given card typically do not impact gameplay directly until said card is used in the game, meaning players need not be concerned with the rules in question until the moment the card is revealed, allowing them to read the additional information.
For example - A tabletop game requiring a large amount of unique enemies would create cards representing said enemies, as each card could easily list different attributes for each monster and may allow space to include additional abilities for an individual monster. Players would not need to memorize 50 different beasts prior to gameplay, and could simply setup a session, only learning the rules of a monster the moment one is revealed in gameplay by reading its card.