QSL cards in amateur radio
Amateur radio operators exchange QSL cards to confirm two-way radio contact between stations. Each card contains details about one or more contacts, the station and its operator. At a minimum, this includes the call sign of both stations participating in the contact, the time and date when it occurred (usually specified in UTC), the radio frequency or Band used, the mode of transmission used, and a signal report. One national association of amateur radio operators, the IARU Region 2, recommends a size of 3½ by 5½ inches (89 mm by 140 mm).
QSL cards are a ham radio operator's calling card and are frequently an expression of individual creativity — from a photo of the operator at his station to original artwork, images of the operator's home town or surrounding countryside, etc. They are frequently created with a good dose of individual pride. Consequently, the collecting of QSL cards of especially interesting designs has become an add-on hobby to the simple gathering of printed documentation of a ham's communications over the course of his or her radio career.
QSL Card
Normally sent using ordinary, international postal systems, QSL cards can be sent either direct to an individual’s address, or via a country's centralized amateur radio association QSL bureau, which collects and distributes cards for that country. This saves postage fees for the sender by sending several cards destined for a single country in one envelope, or large numbers of cards using parcel services. The price for lower postage, however, is a delay in reaching its destination because of the extra handling time involved. In addition to such incoming bureaus, there are also outgoing bureaus in some countries. These bureaus offer a further postage savings by accepting cards destined for many different countries and repackaging them together into bundles that are sent to specific incoming bureaus in other countries.
For rare countries, that is ones where there are very few amateur radio operators, places with no reliable (or even existing) postal systems, including expeditions to remote areas, a volunteer QSL manager may handle the mailing of cards. For expeditions this may amount to thousands of cards, and payment for at least postage is appreciated, and is required for a direct reply (as opposed to a return via a bureau).
Recently, the Internet has enabled electronic transmission as an alternative to the need for mailing a physical card. These systems use computer databases to store all the same information normally verified by QSL cards in an electronic format. Some sponsors of amateur radio operating awards, which normally accept QSL cards for proof of contacts, may also recognize a specific electronic QSL system in verifying award applications.
One such system, eQSL enables electronic exchange of QSLs as jpeg or gif images which can then be printed as cards on the recipient's local inkjet or laser printer, or displayed on the computer monitor. Many logging programs now have direct electronic interfaces to transmit QSO details in real-time into the eQSL.cc database. CQ magazine began accepting electronic QSLs from eQSL.cc for its four award programs in January 2009. 10-10 has been accepting eQSLs since 2002. Another, the ARRL’s Logbook 
of The World (LoTW), allows confirmations to be submitted electronically for that organization’s DX Century Club and Worked All States awards.
Even in the presence of electronic QSLs, physical QSL cards are often fine historical or sentimental keepsakes of a memorable location heard or worked, or a pleasant contact with a new radio friend, and serious hams may have thousands of them. Some cards are plain, while others are multicolored and may be oversized or double paged.
From WIkipedia
What are QSL cards?
When a radio amateur makes contact with another radio amateur for the first time, over the radio, it is customary to exchange a certificate, that the contact has actually taken place. This is the QSL card or verification.
There is a wide variety of QSL cards, and I wanted to show here some of those that seem to me to be more interesting, because they are attractive or unusual.
Very often we, radio amateurs, appreciate QSL cards more that come from an exotic or far away place, where there are few, or no 
resident radio amateurs at all, and not because they are beautiful, or interesting in themselves. Here, QSL cards are displayed which have intrinsic interest, and not because they come from a place with which it is difficult to contact. This is not to underestimate QSL cards collected by DX-ers, for I am a DX-er myself. But this is not the object of this site, which is devoted to the QSL card, and not to DX-ing. (Note: DX is defined as a hard to contact station in a distant country that has few or no resident radio amateurs, and a QSL card from one of these countries is highly prized, because of its rarity).
QSL
QSL is one of the Q codes used in radiocommunication and radio broadcasting. A Q code message can stand for a statement or a question (when the code is followed by a question mark). In this case, QSL? means "do you confirm receipt of my transmission?" while QSL means "I confirm receipt of your transmission". Some also take it to mean "Query Station Location". A QSL card is a written confirmation.
QSL cards confirm either a two-way radiocommunication between two amateur radio stations or a one-way reception of a signal from an AM radio, FM radio, television or shortwave broadcasting station. They can also confirm the reception of a two-way radiocommunication by a third party listener. A typical QSL card is the same size and made from the same material as a typical postcard, and most are sent through the mail as such.
From WIkipedia
History of QSL cards
The concept of sending a post card to verify reception of a station (and later two-way contact between them) may have been independently invented several times. The earliest reference seems to be a card sent in 1916 from 8VX in Buffalo, New York to 3TQ in Philadelphia, Pennsylvania (in those days ITU prefixes were not used). The standardized card with callsign, frequency, date, etc. may have been developed in 1919 by C.D. Hoffman, 8UX, in Akron, Ohio. In Europe, W.E.F. "Bill" Corsham, 2UV, first used a QSL when operating from Harlesden, England in 1922.
From WIkipedia
QSL cards in shortwave listening
Shortwave listeners also collect QSL cards. Sometimes referred to as SWL cards, they can confirm reception of two-way amateur radio communications or commercial radio operators using HF frequencies. A more common form of QSL card for shortwave listeners to collect verifies the reception of signals from international broadcasting or utility stations.
For many international broadcasters, QSL cards serve as publicity tools rather than for gathering data on receptions. Often the cards include information about their stations or countries. Also, announcers may read on the air comments that listeners have put on their own QSL cards.
Other commercial and government television and radio stations have occasionally used QSL card requests as a means of judging the size of their audiences and distances that they can be received. Some of the very early television stations in New York City asked for
listener reports, and HAARP program has occasionally requested reception information on its shortwave experiments, in return for which it sent back QSL cards. Time and frequency stations, such as WWV, will also send QSL cards in response to listeners reports. Other shortwave utility stations, such as marine and aircraft weather broadcasters, may QSL, as do some Pirate radio stations, usually through mail drop boxes.
From WIkipedia