If you're like many people, when you pick out a memory card for your digital camera, you browse through the section to find the most gigabytes for the lowest price. But if you base your SD card purchase on just two factors, you're sorely missing out on one thing: speed. Your camera isn't the only thing that determines how fast you can take pictures. SD card class is a rating that indicates how quickly the memory card can record and save the data.
SD card classes start at Class 2 on the very low end, to Class 10 for the fastest (there's also a UHS Class 1 that's even faster, but it's not compatible with as many cameras). Class 2 cards are hard to come by, and for good reason. Most cameras today take HD video and Class 2 is simply too slow. With more and more cameras having larger sensors and a higher resolution, Class 10 cards have become extremely popular.
While Class 10 is the fastest, not everyone needs to spend that much money for a super fast card. Class 10 cards are excellent for shooting RAW and using the burst mode and are ideal for those shooting with a DSLR or any camera with a large sensor. Class 10 cards are also excellent for using in HD camcorders. If you have no idea what RAW or a burst mode is, then you're likely just fine with a smaller, more affordable card. For small point-and-shoots, a Class 4 or Class 6 card will have plenty of speed. A Class 4 card will offer a good amount of speed for recording the images from smaller cameras, and they're also large enough for recording short HD video clips too.
The more advanced your camera is, the more advanced your memory card should be. The best SD card for DSLRs will be a Class 10, while the best ones for smaller point-and-shoots will be a mid-range class, like Class 4 or 6, because the images are small enough for fast processing without a top-of-the-line card.