You may want to read this article by Canon: http://www.learn.usa.canon.com/resources/articles/2012/eos_understanding_burst_rates.htmlp
The camera has an internal memory buffer. When you take a single shot, that image reads off the sensor and is saved initially into a temporary memory buffer. The camera then feeds that data to the memory card at whatever speed the memory card is able to accept the data. Once the data is completely saved, the internal temporary memory buffer is cleared and the camera can re-use that memory space for more pictures.
It turns out, higher end cameras have larger internal memory buffers. They can usually take quite a few images before the buffer runs out of capacity. This allows you to shoot in continuous burst mode -- even if the memory card being used is quite slow. Of course if the memory card is quite slow then the camera will slow to a crawl while it waits to finish sending each image to that slow card to make more space in the internal buffer to let you resume shooting.
If you have very fast memory cards, the first few images will have been completely saved to the card before the camera gets to the end of the memory buffer... and can immiately start reusing that space from the first few images to shoot even more images (the buffer is literally fillling and emptying at the same time... like a funnel. Put a bigger hole at the bottom of the funnel and you can pour liquid into the funnel even faster without over-flowing the funnel. In this case, the "bigger hole" at the bottom of the funnel is a faster memory card.)
But also keep in mind that JPEGs do not require nearly as much space as RAW images. So while the 5D III's cap on RAW images ranges from 12 to 18 frames (depending on what type of cards you use), if you shoot JPEG and use very fast cards (e.g. UDMA 7 CF cards) the capacity is basically unlimited (the card will run out of space as you shoot thousands of images before the memory buffer actually fills up.) For this reason, some sports photographers will shoot JPEG because they can capture many more frames before they have to pause and wait for the camera to catch up -- even though RAW images capture more detail and offer more adjustment latitude when it comes time to tweak your images on the computer.
If burst speed is important to you, you want to (a) buy the best memory cards you can get (e.g. SanDisk Extreme Pro SD and CF cards, or Lexar 600x SD carsd and Lexar Pro 1066x CF cards).
You do want to get them ONLY from vendors you can trust. Coutnerfeit cards are easy to make (since they're all basically black plastic identical cards and the only thing distinguishing them is the label... it's pretty easy to print a fake label.)
It's fairly common that someone will come to the Canon forum asking for help with a camera that stops recording after a few seconds when shooting video EVEN THOUGH the card they used in the camera is allegedly fast enough to handle the video. In most cases, when we can convince those people to buy a card from a trusted brand and sold through a trusted store, all their problems go away.
I've reached the conclusion that trying to save a few dollars on the cost of memory cards is likely to create more problems than it's worth.
Tim Campbell
5D III, 5D IV, 60Da
