Another difference is that RNA is single stranded while DNA forms a double helix with another strand. The last main difference that I would like to point out is in RNA the base thymine is replaced by uracil. To be honest, I have no idea why, but there is no thymine in RNA.
mRNA, which stands for messenger RNA, is how the genes from our DNA get passed outside of the cell and made into proteins. Since ribosomes, which are made up of rRNA and provide the docking station for tRNA to synthesize a protein, cannot get into the nucleus of the cell and DNA cannot leave the nucleus of the cell, mRNA fixes this problem. The mRNA will bind to the ribosome, and as the ribosome "reads" the mRNA, tRNA's carrying the proper amino acid will dock to it and synthesize the growing polypeptide chain.
Do you think that the amino acid attached to the top of the tRNA can read the mRNA to make sure that it's the correct amino acid or do you think the tRNA is responsible for the correct amino acid?
Also for all you organic chemistry people out there: when in doubt, resonance.