A conventional way of representing a group of cards, usually called a hand, or hands, in text. Useful if you want to avoid stapling 52 cards to each printing of your magazine/newspaper/webpage.
A hand consists of various numbers of
hearts,
spades,
clubs, and
diamonds (in most games, without
duplication -
Pinochle and
Blackjack being notable
exceptions). So, reasonably enough, the display format splits the hand up into
suits, and you get something like:
♠AKQJT ♥5 ♦AKQJT ♣92
Here your holding in a suit is shown to the right of that suit's symbol, and rather than writing out the cards' ranks they are abbreviated to be one character long. Note that, depending on the quality of said publication, the symbols ♠/♥/♦/♣ may be replaced by S/H/D/C - generally because it's less effort than looking up an HTML entity. Also, sometimes the ten is shown as a 10 rather than a T, but T is usually preferable because if suits are shown on top of one another in monospace font it makes it easy to tell at a glance which suits are longest, like this:
♠AKQJT98
♥5
♦AKQ
♣92
This hand, of course, would be interpreted differently depending on what kind of game this hand is dealt for. A bridge player would be delighted: (s)he can make game with no help at all from partner; a hearts player would be violently ill: (s)he will take most of the tricks, but someone will surely use his/her heart and club weakness to give someone else one measly point, thus deflecting a shot at the moon; and a poker player will probably suspect cheating of some kind.
There are a few other instances of shorthand used here, too - being void (having no cards) in a suit is usually represented as ♣-, because it would look confusing if you left out the dash:
♠A42 ♥ ♦KQJT542♣Q85.
Sometimes, people will want to show a hand that they can't quite remember, but they know the important bits of, like, "Well, I had the top five spades, the doubleton ace of hearts, four wimpy clubs, and two small diamonds." A simple expedient is used here: the x character is used to denote, "Some card or other. I don't really remember, and it doesn't matter much."
♠AKQJT98
♥x
♦AKQ
♣xx
The example above makes the point that nobody really cares whether you have the five of hearts or the three of hearts - they're both little and pretty useless. This is mostly a bridge phenomenon, as in many games every card matters: in poker, ♥98765 is very different from ♥98763.
Finally, sometimes you need to refer to a single card, usually to describe the play of the hand. The queen of hearts, then, is usually denoted ♥Q, although some people write it as Q♥. The latter format seems more intuitive (the queen of hearts, not the heart of queens), but the former meshes more cleanly with the format for describing a whole hand.