The winner (more or less) of a relatively low-key format war, the Secure Digital Card, or SD Card, is a removable flash memory device meant to hold data from consumer electronics. Most notably digital cameras and recorders, cell phones, and gaming systems like the NintendoWii and Nintendo3DS.
During the mid 90s with portable devices needing storage starting to emerge, [=SanDisk=] created the Compact Flash format flash device. Being a flat card of about 1.5" by 1", it was quickly met with other competition from Intel, Toshiba, and Sony. By the late 90s, [=SanDisk=] had created another format, the Multimedia Card, to compete with the other offerings as Compact Flash wasn't as compact as the other formats (by then memory cards were about the size of a postage stamp and less than 2mm thick) . Multimedia Card ultimately won out because of its open standard, while everyone else was using proprietary formats. It was also very easy to interface to, being compatible with a variety of well known buses such as the popular SPI embedded bus. The only standard to stubbornly stick to the end was Sony's [=MemoryStick=], which strangely enough was co-developed with [=SanDisk=]. Compact Flash is still around though, mostly enjoyed by professional photographers and hobbyists who found a way to interface it with old computers.
The SD Card evolved from the Multimedia Card. Features included, as the name implies, security features to allow for write and password protection and DRM. Some physical features were added mostly to enhance reliability. Otherwise the two standards are more or less the same
The SD Card has since gained some pretty amazing specifications since its debut in 1999. Micro SD Cards, which are the size of your pinky nail give or take, can hold up to 128 GB, making it the most dense memory device in the world.