"A tradecard is about the size of a 3x5 index card and typically has a nice picture with advertising slogan on the front side, and full advertising text (and sometimes testimonials) on the back side. They were popular between 1880-1900 time period of the Victorian era. Local merchants and street walkers would hand them out for free as a cheap and effective way to advertise products and services. A local store would sometimes stamp their name on the back."

-Antique Bottle Collector's Haven, which has a ton of links to various Trade Card websites