A secure digital (SD) card is a form of a portable, high-performance storage medium available for electronic devices ranging from cameras to PCs.
The Raspberry Pi comes equipped with an SD card slot, allowing us to insert an SD card and use it as our device's main storage mechanism, much like a hard disk on a PC. While you can use other storage mechanisms such as a USB drive or USB external hard drive, the SD card is small and thus lends itself better to embedded devices such as those found in home automation projects.
There are various brands of SD cards in the market, and they come in a wide range of sizes. The Raspberry Pi supports larger SD cards such as those with 64 GB of storage space. For the projects in this book, you should be using a card with a minimum of 2 GB storage.
Note
The official Raspberry Pi website provides a handy guide to SD cards, at http://www.raspberrypi.org/documentation/installation/sd-cards.md.