QSL-cards are cards amateurs exchange to confirm their contact. Originally these are paper cards like ordinary postcards but there are some digital alternatives now like eQSL and Logbook of The World (LoTW).
QSL-cards are primarily send thru the QSL-bureau. This is a world wide network of amateur clubs which exchange QSL-cards. Great thing is it is free to send cards (regardless of the contribution for your membership of the amateur club). Downside is it can take years for QSL-cards to arrive. Not very nice if you are hunting awards and need those confirms to proof your score.
Preferably send me a QSL-confirmation via LOTW, it saves the forest and I got new DXCC’s (countries) confirmed much faster. I’m also a club member so if you want, you can send me a QSL-card via the bureau. If you want a QSL-card as reply, please mention this on your card and I will send you one (image is example of my old card). But I did warn you, it could take a while so please be patient!
73, Jim PA8E