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. In this case, QSL means either "do you confirm receipt of my transmission?" or "I confirm receipt of your transmission". 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.
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.
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 used, the mode of transmission used, and a signal report. One national association of amateur radio operators, the ARRL, 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.
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 verification as an alternative to 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 images which can then be printed and displayed. 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.
An illustrated history of one amateur radio operator's life and QSL collection was published in 2003.
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 Project HAARP 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.
QSL cards are also collected by radio enthusiasts who listen for distant FM radio or TV stations.