QSL cards started out with the ham radio operators. If you had a conversation with someone, you'd swap addresses over the ham radio. Then, you'd each other a card. Many times people would just broadcast their address to see who would send them back a card saying they heard them.
All was fine and wonderful in the world of QSL cards until the CB craze hit the nation. Then the QSL cards got a little weird. You see, ham radio operators look upon their hobby as a craft. You need a federal license to be a ham radio operator. The CBers were looked down upon by the hams as illegal heathens who were defiling the airwaves. The hillbillies had moved in next door.
Many of the CBers were old ham operators. the QSL cards started popping up not only for ham radio users but also for the CB crowd. Most of the ham QSL cards are pretty straight laced. Think of them as the republican guy who manicures his lawn on his knees. The CB QSL cards (which I mostly collect) are like a big trailer park decorated with plastic flamingos, tiki torches and 1978 El Caminos. You'll see outhouses, mobile homes, sexual innuendo and all sorts of insanity on the CB version of the QSL cards.