SD cards are designated with different speed classes based on the read and write speeds they can deliver. Typically, the speed class of a SD card is printed on the exterior of the card. To find out the speed class, look for the speed class symbol on your SD card as shown here.
If your SD card does not have any speed class symbol printed on it, do not despair. Plug in your card and figure out the read and write speed of the card as explained here. Then compare the write speed with the minimum write speed guaranteed by different speed classes as shown in the table here. This is pretty easy to remember: a class 2 device promises a write speed of at least 2 MB/sec, a class 4 device promises a write speed of at least 4 MB/sec and so on.
Tried with: Ubuntu 12.04 LTS