Secure Digital card
There are also different classes. These refer to the read and write speeds. Currently in use are Class 2 - which means 2 MBytes/second, Class 4 - 4 MB/sec and Class 6 at 6 MB/sec. The problem with this is that different manufacturers measure different things with it, some measure write speeds, others read speeds. There are also differences because some give guarantees, and others simply say that the speeds they indicate are the maximum achievable under good conditions.