The class rating system has its limitations, but it can be a handy guide to the practical capabilities of different cards. A class 2 rating means the card is guaranteed to be fast enough for standard-definition video recording, while classes 4 and 6 are fast enough for Full HD video (which one you need will depend on the bit rate of the video format you’re using).
The highest rating, class 10, is faster than required for any modern video standard: rather, it’s aimed at stills photographers. The idea is to minimise the time it takes to write a photograph to the card, so you can take multiple shots in rapid succession without having to wait around for each one to be stored.
It may seem counter-intuitive that capturing still images requires a faster card than shooting video, but Full HD footage isn’t as space-hungry as you might imagine. Despite the “high-definition” terminology, each HD frame has a comparatively low resolution of just over two megapixels. Plus, since consecutive frames of a video are often extremely similar, clever compression techniques can be used to store moving images efficiently. A data rate of 4-6MB/sec is ample for continuous shooting.