Wikipedia
The MultiMediaCard (MMC) is a memory card standard used for solid-state storage. Unveiled in 1997 by SanDisk and Siemens AG, it is based on a surface contact low pin-count serial interface using a single memory stack substrate assembly, and is therefore much smaller than earlier systems based on high pin-count parallel interfaces using traditional surface mount assembly such as CompactFlash. Both products were initially introduced using SanDisk NOR-based Flash technology. MMC is about the size of a postage stamp: 24 mm × 32 mm × 1.4 mm. MMC originally used a 1- bit serial interface, but newer versions of the specification allow transfers of 4 or 8 bits at a time. MMC can be used in many devices that can use Secure Digital (SD) cards.
Typically, an MMC is used as a storage medium for a portable device, in a form that can easily be removed for access by a PC. For example, a digital camera would use an MMC for storing image files. With an MMC reader (typically a small box that connects via USB or some other serial connection, although some can be found integrated into the computer itself), a user could copy the pictures taken with the digital camera off to his or her computer. Modern computers, both laptops and desktops, often have SD slots, which can additionally read MMCs if the operating system drivers can.
MMCs are available in sizes up to and including 512 GB. They are used in almost every context in which memory cards are used, like cellular phones, digital audio players, digital cameras and PDAs. Since the introduction of SD cards, few companies build MMC slots into their devices (an exception is some mobile devices like the Nokia 9300 communicator in 2004, where the smaller size of the MMC is a benefit), but the slightly thinner, pin-compatible MMCs can be used in almost any device that can use SD cards if the software/firmware on the device is capable.
While few companies build MMC slots into devices today (SD cards are more common), the embedded card (eMMC) is still widely used in the industry as a primary means of integrated storage in portable devices. It provides a low-cost flash memory system with a built-in controller that can reside inside an Android or Windows phone or low-cost PC and appear to its host as a bootable device, in lieu of a more expensive form of solid-state storage, like the traditional solid-state drive.