SD cards are small thumbnail-sized memory storage cards.
There are several varieties of SD cards available today with max capacities ranging up to 128 TB.

An SD card, short for Secure Digital card, is a type of removable memory card used to read and write large quantities of data in a wide variety of mobile electronics, cameras, smart devices, and more.

It has become the default standard of memory card for most consumer electronics (though a smaller version, the Micro SD card, is commonly used in phones and other devices where physical space is more of a premium).

What to know about SD cards
The SD card debuted in 1999 and is the successor to the now-obsolete MultiMediaCard (MMC). It was one of a number of competing memory card formats in use by consumer electronics, such as Sony’s defunct Memory Stick and the CompactFlash card, which, while still in use, is much less common than it was in decades past.

As the electronics industry has evolved, the original SD card specification has been updated several times to allow for higher capacities. That means there are several types of SD cards sold and used today:

SD: The original SD card is no longer produced. It had a maximum capacity of 2 GB and used the FAT16 file system.
SDHC: The Secure Digital High Capacity (SDHC) card debuted in 2006. It supports capacities up to 32 GB and uses the FAT32 file system.
SDXC: The Secure Digital eXtended Capacity (SDXC) format got its start in 2009. It can store up to 2 TB on the exFAT file system.
SDUC: Secure Digital Ultra Capacity (SDUC) cards appeared in 2018 and can support 128 TB per card using exFAT.

Common uses for SD cards
Due to their relatively small size, SD cards have been widely adopted in an array of consumer electronics for inexpensively storing gigabytes of data. They’re found in digital cameras, video game consoles, set-top boxes, and home theater devices like Blu-ray players, as well as smart home/Internet of Things devices like security cameras. While early smartphones used SD cards, most phones have migrated to the smaller Micro SD card format.