The speed rating measures maximum transfer speed for writing and reading images to and from the card, expressed as megabytes per second. However, video doesn't need as big a pipe because the video format is a smaller "fixed stream" that uses only a portion of the pipe.
Unlike card write speeds that measure maximum performance, class ratings measure the minimum sustained speed required for recording an even rate of video onto the card. The class rating number corresponds to the transfer rate measured in megabytes per second. Class 2 cards are designed for a minimum sustained transfer rate of 2 megabytes per second (MB/s)1, while Class 10 cards are designed for a minimum sustained transfer rate of 10MB/s2.
What does this difference mean for me?
Rated Speed (e.g. 15MB/s, 30MB/s, etc.) is maximum speed of the card and also what you would expect to approximately see in typical usage of writing or reading files on the card. This measurement is pertinent to still photography, especially for taking pictures with high resolution and/or saving in RAW format where the files created are very large. The faster the card, the faster it can save the file and be ready to take another picture. You really notice speed differences with high-megapixel DSLR and multi-shot burst mode.
Still digital images shot on high-megapixel cameras should utilize fast data throughput (a large pipe), higher speed cards for improved performance. Higher speed cards can also improve how fast you can download (or upload) the files from the card to your computer.
Speed Class is a minimum speed based on a worst case scenario test. The Speed Class is important for video mode or camcorders, where the device is actually saving a steady stream of data. The resolution and format of the video determines the amount of steady stream data. This translates to a minimum speed you need to guarantee that the video captured on the cards is recorded at an even, sustained rate with no dropped frames (which typically results in lost data and choppy playback).
Compared to high-megapixel photography, video doesn't need as big a pipe because the video format is a smaller "fixed stream" that uses only a portion of the pipe. But you do need a minimum guaranteed speed for the SDHC card that satisfies the requirement of the data stream. Your camera's specifications should state the minimum SDHC Class Rating required.
Using a card without the proper class rating on a more advanced camera, such as a high-definition (HD) camcorder or Digital Single Lens Reflex (DSLR) camera with HD video record settings is likely to result in an error message indicating that video can only be recorded at a lower definition setting.