RNA is short for RiboNucleic Acid. It is similar to DNA (DeoxyriboNucleic Acid), except the Thymine in DNA is replaced by Uracil in RNA. Its primary function in the cell is to code for the amino acids that build proteins. It also plays a role in regulating the various functions of a cell.
RNA is also structurally different from DNA. Instead of being a double strand, RNA only exists in single strands. However, although DNA is limited in form to a double-helix, RNA's chemistry allows the single strands to exist in many different shapes.
Some viruses consist of a strand of RNA instead of DNA. They work by "hijacking" the cell's DNA to create more copies of the RNA and more protein for the virus's coating.