The earliest business cards were believed to have originated in China, sometime in the 1400's. Back then, they weren't actually cards for business, they were considered visiting cards. If an individual wanted to meet with someone else, possibly a man of business or of some importance, they would send along their card first, as a sort of a pre-introduction.
Now-a-days, in business, this is called a pre-approach letter. A letter that you would send to someone as a preliminary introduction - prior to actually attempting to set up an appointment for a meeting.
Eventually the custom reached Europe, possibly by the1600's, and became the custom of the day with the elite. Wealthy people would have their own engraved cards printed and would present them at the door as they visited new acquaintances for the first time. The maid or butler who answered the door would then take the card to their mistress or master, and she or he would decide it they wanted to grant admittance to the presenter of the card.
Eventually the custom spread to businesses, and colorful trade cards were printed on both sides that would advertise products, in order to induce people to buy them, much as radio or television or newspaper advertising does today.
As the popularity of trade cards continued into the 1800's and early 1900's, so did the evolution of the business card as it has come to be known today. All sorts of tradesmen and professional business men had business cards printed, in order that they might present them to prospective customers as an introduction of themselves and the goods or services that they could provide.
Todays business card are sometimes printed on both sides, if there is an usually large amount of information to be imparted. But usually, today's business cards are printed on one side only, often with a logo, or small picture in the top left or right side of the business card. They can be printed in one or more colors of ink, in different type styles and sizes and also be printed on different types of card stock.
Much as the original business cards, today's modern business cards serve as an introduction to new prospects, that explain who you are and what goods or services you want to provide to them.