http://www.biology.ucsc.edu/rnacenter/what.shtml
RNA and DNA are both nucleic acids. In order to describe the structure and function of RNA, it is helpful to review its structure relative to that of DNA and how their structures determine their functions. RNA stands for ribonucleic acid, a close chemical relative of deoxyribonucleic acid, or DNA, the molecule that carries the unique instructions for making cells, the fundamental living units of all organisms. DNA carries this information in strings of genes. If all the DNA in the cell (the genome) is thought of as a library of information to build that cell, then each gene can be thought of as a book in the library. While human DNA from different individuals is basically similar in the genes it contains - after all, humans are built to the same basic design - there are variations within genes that can be thought of as analogous to changes in the wording of some of the pages of the books. These minor changes within the genes make the DNA in any given person unique.
DNA has been in the news recently because the method of DNA fingerprinting can be used to determine the source of a DNA molecule, and because of The Human Genome Project-national and international programs aimed at obtaining the complete sequence of human DNA. However, while most people have heard of DNA, they are less familiar with RNA. Like DNA, RNA can also store information and is essential for carrying out the instructions stored in DNA.
RNA is present in several different forms in living cells. The first form - messenger RNA or mRNA - is copied from the DNA "blueprint" and used as a template for assembling proteins from their amino acid building blocks. However, amino acids cannot be lined up directly on the mRNA to be assembled into proteins. Two additional RNA molecules are required for this process. Transfer RNA or tRNA is used to "read" the amino acid sequence on the mRNA and assign the correct amino acid into the growing protein chain. This process requires a specialized machine - the ribosome. The ribosome itself is built of a collection of proteins folded with a third group of RNA molecules - ribosomal RNAs or rRNAs. Primarily through the pioneering work of Harry Noller, it is now thought that the rRNA acts not just as a scaffold to hold the proteins of the ribosome in place, but is directly invoved in joining the amino acids together in the growing protein chain. Other machinery in the cell also uses protein and RNA complexes. An important example is the spliceosome, the machinery that removes extra sequences from mRNA during maturation. Finally, as was indicated above, RNA is also used to store information in some viruses, in a manner analogous to DNA. A relevant example is that of the Human Immunodeficiency Virus or HIV. The genes of HIV are carried in an RNA molecule and the host cell rRNA and tRNA molecules are used during the course of HIV infection.