Cabs

Hey all, I was just wondering if a marshall 120watt cab ( its actually a marshall MG412-A 120watt cab) is connected to my marshall 30MGDFX amp would it make any differnce to the sound/ volume, would it increase the volume?

I dont really know about cabs and I was just wondering also if any one can give me any info, are they just an extension to the speaker??

thanks alot,

\m/ \m/ metal up your ass