This post is concerned on the most widely used External Memory Devices "micro SD". Some of my friends asked me that "Why are there big differences between the same sized memory cards of the Same brand in the same store?
Actually this all is the Game of speed and reliability.
Technically microSD cards can be divide in 4 Classes.
Class 0, Class 2, Class 4,Class 6, and the fastest available Class 10.
Class 0 - These cards do not specify performance, which includes all legacy cards prior to class specifications.
See the wikipedia chart for more details.
Have a look.