Secure Digital (SD) cards are memory cards used extensively in portable devices, such as mobile phones, digital cameras, GPS navigation devices, handheld consoles, and tablets. It is a family of solid-state storage media.
The Secure Digital format includes four card families. The four families are the original Standard-Capacity (SDSC), the High-Capacity (SDHC), the eXtended-Capacity (SDXC), and the SDIO, which combines input/output functions with data storage. SDHC and SDXC are the most common.
SD cards are usually differntiated by their speed. Each card has a speed "class" indicating minimum performance to record video - both read and write speeds must exceed the specified value. Classes range from 2 up to 10, followed by U1 (which is the same speed as 10 but uses an updated connection type) and U3.