RNA is important to cells because it relays information encoded in DNA to tiny organs within the cell, called ribosomes, which produce protein according to the RNA's instructions. RNA is thus vital to the basic functioning of the cell.
DNA contains the master instructions for a cell. The information is encoded in small chunks called codons, which form groups called genes. Collections of genes form larger groups called chromosomes. Chromosomes are found within the nucleus of a cell.
DNA is so important to the functioning of a cell that it is heavily protected within the nucleus. If the information it contains gets damaged, it can result in mutations that can harm the cell, kill it or even cause cancer. However, DNA is so heavily protected that there is no direct way for the instructions to be sent.
RNA is the biochemical messenger between the nucleus and the protein factories called ribosomes, which are found floating in the cell's fluid. Some RNA, called mRNA, copies information codons from the nucleus and transports the message to nearby ribosomes. Another kind of RNA, called tRNA, helps in copying the message off of the mRNA on to the ribosome so it can make use of the information. A kind of RNA in the ribosome, called rRNA, forms chains between amino acids via peptide bonds, building the acids into protein.
RNA is also important in gene expression and regulating chemical reactions within the cell. Micro RNA, or miRNA, can inhibit the expression of genes by binding to mRNA. Ribozymes are RNA molecules which act as enzymes, speeding up biochemical reactions.
Finally, viruses called retroviruses use RNA rather than DNA as their master instructions. One such example of a retrovirus is HIV.