The first of the two speed classes is called "Speed Class".
These chips are identified by a 2,4,6 or 10 inside a "C" shape as shown below. Class 2 cards are getting rare because they really arent much faster than an unmarked card.
You may also see them listed as Class 4 ect.
These are the most common of the rated cards and can be used interchangeably. They are also interchangeable with ?x rated cards and unrated cards.
 | Class 2 Class 4 Class 6 Class 10 |
What does it mean ?
This number tells you how fast the cards controlled can handle reading and writing data. It is measured in MB per second.This one is very simple a class 2 can handle a minimum transfer rate of 2 MB per second, Class 4 is 4 MB per second and so on up to class 10 which can handle 10 MB per second.
How fast a card do I need ?
For music any card will do.
For a larger Mega Pixel Camera a class 4 or 6 will let you shoot faster and reduce the time you see that hourglass as your picture is stored.
Video is where these shine , For HD video a level 4 or 6 minimum is reccomended. and a ten would be better.
Buying Tips
If the price is the same Ill get the fastest card. If you arent doing Video or recording huge photos I wouldnt spend extra for a faster card.
