RNA (Ribonucleic acid) is used along with DNA in genetics. While DNA is the manner in which the organism's genes are stored (with the exception of some viruses which use RNA for that purpose), RNA is used in "transfer" and "messenger" functions to transmit genetic information where it is needed in biological processes. RNA has four bases, guanine, adenine, uracil and cytosine, which are coded as G, A, U, and C respectively. Three of these are the same as those in DNA, but uracil (U) is used in place of the thymine (T) found in DNA. With this one-on-one correspondence, all the information contained in DNA can be encoded in RNA and vice versa.
There are theories to the effect that RNA preceded DNA as the evolutionary means of storing and transmitting genetic information, due to the fact that, unlike DNA, RNA molecules can "fold" in a manner akin to that of proteins, allowing biological organisms to be built up entirely of RNA, albeit not in as complex and stable a manner as modern-day "life as we know it" which separates the functions of genetic material and protein folding.