SD is an abbreviation for “secure digital.” The data on an SD card is stored in a digital format. The solid-state chip inside the card’s plastic cover contains many tiny electrical circuits. When the card is not in use, the circuits retain their charges without any additional power. When a card is placed in an activated device, such as a camera or a cellphone, a small electrical current from the device moves electrons in the flash memory chip. The digital patterns stored on the chip correlate to the data stored there. Data is erased when a slightly higher voltage is applied to the circuit. This allows for rewriting. SD cards write and erase memory in blocks or sections. This makes them faster than some other varieties of data storage.