What is a QSL card? Why would I ever receive one? Why would I ever bother to send one? If so, when? Is it rude not to?
Are they postcards sent by physical postal mail, or are they now digital thingys?
A QSL card is a physical card usually the size of a postcard. They are sent to verify a contact known as a QSL. To get various awards like Worked All States or DXCC (worked 100 entities) you need to verfiy the contacts. The main way this used to be done was to take the QSL cards you had received to a "card checker" who would verify them.
The card should have your information including call sign, the call sign of the person you contacted, the date and time of the contact, band or frequency, mode (SSB, FM, etc), and a signal report.
Today most verification is done using online services like Logbook Of The World or LOTW, but some people still like to use QSL cards.
Within a country they are usually mailed directly. I contacted you so I send a QSL card with the contact information. You can then send me back your QSL card confirming the contact.
Outside of the country we often use a QSL Bureau. we send our cards to them. they gather all of the card going to a country and send them to bureau in that country that then send them to individuals. To receive QSL cards from your bureau you need to supply with stamped self addressed envelopes. Cards often arrive months after the contact, but this is the least expensive way to send them and receive them.
Not every country has a QSL service Bureau. With the disappearance of international reply coupons you could use for postage it is more difficult to send QSL cards directly to people outside of your country. Some suggest sending a 'green stamp', a US Dollar, to help cover postage.
Many QSL cards have colorful art work or photographs. Others are very plain. They can be a way for you to creatively identify yourself. Many Hams keep a book or board with QSL cards they have received.
The TV show "last man standing" features Ham radio from time to time. You will on the wall QSL cards that people have sent to the show after making a contact with the cast or crew using the station. People watch the show to see if they can spot their card.
It should also be noted that many hams now upload their logs to Log of the World, or other services, and that contest award providers can use these services to match up contacts, making QSL cards a bit less required than they previously were. Nonetheless, they are still very nice to have as a tangible reminder of an accomplishment.
It's also good to know that non-US hams may not be able to afford to keep up with QSL requests without being sent several dollars to cover costs.