The higher the class number the faster the write speed, most commonly used for video cameras.
With Android Phones read speed is most important. The faster the read speed the faster your phone fully boots up ( the ability to use all apps and functions) and your SD card is read and fully functional.
Read speed is not regulated and can vary widely. One manufacturer's class 4 might have a better read speed than another's class 10.
Currently the Sandisk class 4 16GB micro SD card has the best read speed, averaging a little over 40MB/s
The average SD card is in the 25MB/s range
Note: These values are with an empty card and will come down a few MB/s as the card is filled.
EDIT: SD Tools is an app found in the PlayStore that test the read and write speed of an SD card. Maybe members with different Manufacturer SD cards and classes could post their Card Info plus read/write speed in case there is a micro SD card that has a better read speed of the Sandisk I described?