An SD card, which stands for "Secure Digital" card, is a small, portable storage device that is commonly used to store data on a wide range of devices, such as cameras, smartphones, tablets, and laptops. It is a type of flash memory card that uses non-volatile memory to store data, which means that the data remains intact even when the power is turned off.

SD cards come in different sizes, the most common being the standard size SD card and the smaller microSD card. The standard size SD card has a size of 32mm x 24mm x 2.1mm, while the microSD card is much smaller at 15mm x 11mm x 1mm. The majority of microSD cards are sold with an SD card adapter which allows microSD cards to be used in full-size SD card slots.

SD cards also come in different speed classes, which are indicated by a number and a letter inside a small circular logo on the card. The most common speed classes are Class 2, Class 4, Class 6, and Class 10. Each class represents a minimum data transfer rate, measured in megabytes per second (MB/s), that the card is capable of sustaining. For example, a Class 10 card can transfer data at a minimum rate of 10 MB/s, while a Class 2 card can only transfer data at a minimum rate of 2 MB/s.

In addition to speed classes, SD cards also have UHS (Ultra High Speed) speed classes, which are designed for higher-performance devices such as 4K video cameras. The UHS speed classes are UHS-I, UHS-II, and UHS-III, with each class offering progressively faster data transfer rates.

SD cards are widely used for storing and transferring data such as photos, videos, and music files. They are commonly used in cameras and other mobile devices to store images and video recordings. They are also used as a storage medium for music players, gaming consoles, and other portable devices.

The main brands of SD cards include SanDisk, Kingston, Samsung, Toshiba, and Lexar, among others. These brands offer SD cards in a wide range of sizes, speeds, and capacities to meet the diverse needs of users.