What are the functional differences between RNA and DNA

DNA is the medium which encodes the information that makes an organism what it is. DNA usually forms a double stranded structure and encodes genes and other regulatory elements. 

RNA can act as either catalyst like proteins, a store of information like DNA, or both. mRNA is produced when the DNA of a cell is read by specific enzymes (RNA polymerase) and is then itself read by ribosomes to produce proteins. Hence earning its name of messenger RNA.