A j-card is the paper card inserted in the plastic storage case of most pre-recorded audio cassette releases, as well as cassettes of bootleg or copied concerts of the jam band scene. The j-card usually contains an image of the album cover, a track listing, credits, and copyright information.
The j-card gets its name from being folded into the shape of the letter J (when viewed from the side) to fit inside the cassette's case.
Today, custom made CD-R covers are often called J-cards. J-Card style covers were used in CD's covers in the 90's for single cases when bands release singles and are still used today.