Taken from
HERE
The SD Association has created and defined 3 speed
classes to help you identify speed and performance
capabilities/minimum requirements of SD/SDHC cards
and the host products. The SD Speed Class Ratings
specify a minimum sustained write speed for SDHC
cards (Class 2: 2 MB/s; Class 4: 4 MB/s; Class 6: 6
MB/s*).
if you shoot in burst mode, you want a faster card really, so the buffer won't run out as quick.