hai there, can you explain to me what is the difference between class 4, class 2 and class 6 MicroSDHC.

And can you tell me what is the price item below?
16GB Class6 MSPD : CR5400 + Transcend 8GB Class6 MicroSDHC + Transcend 8GB Class6 MicroSDHC
16GB Class4 MSPD : CR5400 + Kingston 8GB Class4 MicroSDHC + Kingston 8GB Class4 MicroSDHC
16GB Class2 MSPD : CR5400 + Sandisk 8GB Class2 MicroSDHC + Sandisk 8GB Class2 MicroSDHC