I get asked this a lot from friends and clients - both for their Digital Cameras as well as their Video Cameras that capture to an SD Card.
The speed of an SD card determines how quickly your camera/device can write data to the card (and how quickly it can play back the data as well). This is measured and provided by the manufacturer and is marked on each card (and often the package) by a letter C that almost looks like a circle with the speed numeral inside it.
If you are capturing single photos on your digital camera you could get by with a Class 4 card but that will definitely not work well if you are shooting in Burst Mode (capturing multiple images - like sports) or in HD video mode.
If you are using either of those features on your camera a Class 6 card should be sufficient - though I tend to err on the side of caution and recommend using a Class 10 card.
By the way, SD is the acronym for Secure Digital, the specific flash media device that records data. Today we have not only SD but also SDHC (SD High Capacity) and SDXC (SD eXtra Capacity).
