QSL-cards are cards amateurs exchange to confirm their contact. Originally these where paper cards like ordinary postcards. There are some digital alternatives now like eQSL and Logbook of The World (LoTW).
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 (see image for example of my card).