SD - The original SD card. SD stands for "Secure Digital" and was developed by the SD Card Association with Panasonic, SanDisk, and Toshiba. These should be avoided since SDHC and SDXC cards (below) are faster and provide larger capacities.
SDHC - SDHC stands for "Secure Digital High Capacity." These cards range from 4GB to 32 GB. Most devices, especially newer ones support the SDHC specifications.
SDXC - SDXC stands for "Secure Digital Extended Capacity." These cards were unveiled at last year's 2009 CES tradeshow and are the latest in SD specifications. Currently, only newer Panasonic cameras and the new Canon EOS Rebel T2i digital SLR support this format. SDXC cards range have capacities in excess of 32 GB and can sustain transfer rates at 104 MB/second (MB/s) with future cards reaching 300 MB/s.
Class Speeds - Classes 2, 4, 6, and 10 cards have 2, 4, 6, and 10 MB/s transfer rates. Some cards specify actual speeds like 20 or 30 MB/second read or write speeds. Obviously, the faster the better. Make sure to compare like specs for accurate comparisons. In general, stick with Class 6 and higher for video recording.
X Ratings - X speed ratings are similar to class speeds. To find the transfer rate in MB/s , simply multiply the number in front of the X by .150 KB/second. So a 133x card will yield a rate of 20 MB/sec.