Not all flash cards are created equal: They have different read and write speeds specifications. For still shooters, the write speed of the flash memory card may limit how fast the camera clears the buffer and allows the camera to shoot again. For video shooters, the sustained write speed is critical to capture videos at high bit rates. The read speed of the cards matters too. It determines how fast you can download the photos to your computer.
Should you buy the fastest card then? Not necessarily. It all depends on the actual requirements of your camera and your need. Faster cards do cost more. The following video has some examples of how to determine the required flash memory card speed based on the maximum video bit rates. Hopefully by the end of the video if can understand what “Class 4” and “600x” represent and choose the proper card for your camera.
Sometimes, the labeled speed spec may not be accurate. So you should buy a card with some margin in the write speed spec over the maximum write speed your camera needs. You may also want to check out this SD/CF performance database.