There are three main differences between DNA and RNA: 

1. RNA has the base uracil instead of the base thymine. These bases are very similar and differ by only one chemical group!

2. RNA nucleotides have a ribose sugar instead of a deoxyribose sugar like DNA. Having an extra OH groups mean that RNA nucleotides are more likely to suffer nucleophilic attacks. This is one of the reasons RNA is more unstable than DNA.

3. Usually in the cell DNA forms a double stranded helix, whereas RNA is usually single stranded. Cells make all kinds of weird and wonderful RNA molecules, some of these do have secondary structures and regions of double strandedness. For GCSE you learn about tRNA for example, this RNA folds back on itself and so has double stranded regions.