RNA virus

RNA viruses are viruses that use RNA as their genetic material. In humans, well-known RNA viruses are those that cause SARS, Influenza or Hepatitis C.
Viruses that are not RNA viruses use DNA for their genome and are called DNA viruses.
Viruses have a classification system, invented by David Baltimore, a Nobel Prize winner. The RNA viruses in the Baltimore system are classified as:
"Sense" in RNA means "ready to make proteins, as messenger RNA does". "Antisense" in RNA means "complementary to messenger RNA". In complete complementarity each nucleotide is across from its opposite number, so antisense RNA can produce sense RNA. 
Another system is run by the International Committee on Taxonomy of Viruses (ICTV). It uses terms from the familiar Linnaean taxonomy, like orders and families.
Mutation rates.
RNA viruses have very high mutation rates compared to DNA viruses. This is because viral RNA polymerases lack the proof-reading ability of DNA polymerases. This explains why it is difficult to make effective vaccines to prevent diseases caused by RNA viruses.
Some genes of RNA virus are important to the viral replication cycles and mutations are not tolerated. For example, the bit of the hepatitis C virus genome which encodes the core protein is highly conserved, because it is essential to start translation.