Before CDs came along, audio cassettes were typically the portable audio format of choice in non-professional environments. The devices consist of two miniature spools for a magnetically coated plastic tape upon which the audio is encoded, protected by a simple plastic shell.
As A Storage Medium
Throughout the 1980s, many systems read data off of these cassettes in an audio format, allowing the use of standard-issue tapes as a cheap and efficient format for game data. While microcomputers like the ZX Spectrum and Commodore 64 were famous for this, other methods of gaming experimented with this as well - Data East's DECO Cassette System used the tapes to hold data for arcade games.
Ultimately, though, cassettes suffered from lengthy load times, and were slowly discarded in favor of floppy disks and ROM cartridges.