SD cards range in storage capacity from 4 Megabytes up to 2 Gigabytes. 4Gb SD cards exist but these violate the standard and unsurprisingly they cannot work in some devices. Standard card dimensions are 32 x 24 x 2.1mm. A range of speeds are available; standard cards transfer data at around 900kB per second, but high-speed versions can run at 10MB per second, with some capable of much higher rates.
Two extensions to the standard, SDHC (High Capacity) and SDXC (eXtended Capacity) raise the size limits to 32 Gigabytes and 2 Terabytes respectively.
Mini-SD (21.5x20x1.4mm) and Micro SD (15x11x0.7mm) cards use the same technology, in a smaller packages.