Wikipedia
The Visual Memory Unit (VMU), also referred to as the (VM) in Japan and Europe, is the primary memory card produced by Sega for the Dreamcast home video game console. The device features a monochrome liquid crystal display (LCD), multi-player gaming capability (via connectors at the top), second screen functionality, a real-time clock, file manager, built-in flash memory, and sound capability. Prior to the launch of the Dreamcast, a special Godzilla edition VMU, preloaded with a virtual pet game, was released on July 30, 1998 in Japan, 6 months before Sony released a similar device (albeit lacking any of the VMU's second-screen functionality) for its PlayStation console.
While its most basic function is as a removable storage device, the VMU may also serve as an auxiliary display during normal gameplay and, through the use of additional software (distributed as extras on Dreamcast GD-ROMs), acts as a handheld game console. Console-like features of the VMU include a screen, speaker, proper directional pad, four action buttons, the ability to connect and interact with other VMUs, and the ability to download additional games. While the standard VMU is white, colors were expanded to include many variations. Japan even saw the release of branded VMUs, such as those by Sonic Team, Capcom and Hello Kitty.