SD (Secure Digital) cards and their associated SC/HC/XC technologies are the primary storage medium for the Raspberry Pi. The Pi Models A and B use SD Cards, and new models use Micro SD. SD cards are non-volatile flash memory.
Secure Digital (SD) cards are non-volatile memory cards, which were originally designed for portable devices, such as cameras, phones and tablets.
To minimise start-up costs, the Raspberry Pi utilises SD cards as its primary storage medium.
This decision does have its down sides, SD cards have comparatively slow read/write speeds and can have relatively short life spans.
Operating Systems for the Raspberry Pi are 'flashed' to the SD card, meaning that complex installation procedures can easily be skipped.
Working/Problem SD Cards
Not all SD Cards are compatible with the Raspberry Pi. The below links are a list of both working and problem SD Cards.
SDHC (SD High-Capacity) are 4 GB-32 GB.