DNA and RNA are related molecules: they are both nucleic acids. But where as DNA has two strands, twisted together into a double helix, RNA usually occurs as just one strand. Both use combinations of four bases to store their information ΓÇö but they do not use the same four. In place of thymine, RNA uses a base called uracil.
Many people think that there was an RNA world before the arrival of DNA. Life then, such as it was, was based on RNA alone. But DNA is more stable than RNA. This could be one reason that most organisms, including many viruses, store their genetic information as DNA.
Some viruses, however, known as retroviruses, store their genetic information as RNA instead. These viruses (HIV, the virus that causes AIDS is one) have extremely high mutation rates, changing all the time.
