What are two ways in which RNA structure differs from DNA structure?

Chemically RNA and DNA only differ by a single oxygen atom in each nucleotide. Specifically the sugar group in an RNA nucleotide is the sugar ribose, wherease the sugar group in DNA is deoxyribose.

Wikipedia has a nice image showing this here: http://en.wikipedia.org/wiki/File:Nucleotides_1.svg

Structurally RNA and DNA are also different. DNA exists almost exclusively in a double stranded helix. RNA is typically thought of a single chain that has a far more chaotic structure with the RNA folding back onto itself creating small helical regions where possible.

DNA nucleotides contain a different sugar than RNA nucleotides.