An aperture card was a type of punched card into which a piece of microfilm was mounted. They were introduced in 1943 and initially used by the US military to store photographs of strategic value.
The card itself contained metadata about the image punched into the card, along with information printed across the top of the card for visual identification. The microfilm was usually on 16 or 35mm film, and contained a single image of a document, typically an engineering drawing.
Machines could be used to sort and retrieve specific cards. More recently, machines have become available to scan and digitise remaining aperture cards, and the contents of 3000 aperture cards can be stored on one CD-ROM.