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.