We have that DNA is a double-stranded molecule made up of deoxyribonucleic acid, while RNA is a single-stranded molecule made up of ribonucleic acid. What does this mean? That refers to one of their main compounds, the pentose sugar, which has one Oxygen atom not linked (deoxyribose; DNA), or linked (ribose; RNA) to the second Carbon atom, forming an -OH group in that position.
Another difference we could mention can be that regarding the nitrogenous bases ( Adenine (A), Guanine (G), Cytosine (C), Thymine (T), and Uracil (U) ), specially Thymine and Uracil, which are proper to DNA, and RNA, respectively.
In the same way, one more difference between these two molecules is that they´re used for different purposes. DNA is involved in the storage of genetic information, while RNA is involved in protein synthesis.