Secure Digital, or SD, is a non-volatile memory card format that is often used in portable devices such as PDAs, cell phones and smartphones, digital cameras, handheld computers, media players, GPS receivers, and video game consoles.
Standard SD card capacities range from 4MB to 4GB. Higher capacity SDHC cards range from 4GB to 32GB as of May 2009. The latest SD format, called SDXC (Secure Digital eXtended Capacity), was announced at the 2009 Consumer Electronics Show. SDXC will allow for 2TB capacity cards.
SD is a very popular format, however it's important to realize that the newer formats are not compatible with old devices. Thus, if you have a PDA that can accept SD cards, it is unlikely the handheld can also accept SDHC cards. However, SDHC cards are backwards compatible with SD cards. In other words, if your PDA accepts SDHC cards, you can also use SD cards in the handheld.
SD and SDHC cards have the same physical shape and form factor, which has created some confusion for consumers. There are also miniSD and microSD cards which are similar but smaller in physical size. Many miniSD and microSD cards come with adapters that convert the smaller card into a full-size SD card.