The "SD series" refers to a line of memory cards known as Secure Digital cards. These cards are widely used for storing data in devices such as cameras, smartphones, and tablets. The SD series includes various types, such as SD, SDHC (Secure Digital High Capacity), and SDXC (Secure Digital eXtended Capacity), each offering different storage capacities and speed classes. The SD series was developed by the SD Association to provide a standardized format for portable storage. These cards are popular due to their compact size, reliability, and compatibility with a wide range of electronic devices. They are essential for users needing extra storage for photos, videos, and other digital files.