Pogs were a game, a trading card, and a souvenir all rolled into one. They were very popular among kids during the late 1980s and early '90s. But they have a history that goes back much further.
The actual gameplay behind pogs has long been attributed to the classic Japanese game of Menko (above), which has been popular since the Edo Period (between 1603 and 1867) and also centered on players attempting to flip the cards or pieces of their opponent. Much like modern pogs, the original Menko playing pieces were roughly the size of milk caps and featured images of Japanese cultural icons, like wrestlers and warriors. These pieces weren't made out of cardboard or plastic, but shaped from clay, wood, or ceramics (though Menko later included cardboard pieces that are considered the forerunners of trading cards).
But how did pogs become a huge fad in the late 20th century? The rest of the story, including how they came to be called pogs, is at mental_floss.