RNA, like
DNA, is composed of
nucleotides, as mentioned previously by
January. There are two variations between RNA and DNA, though, that make RNA much less stable:
1. DNA has 2'-deoxyribose for its sugar base; RNA has ribose. Both are pentose sugars, but the lack of oxygen in 2'-deoxyribose prevents that carbon from being used in bond formation (both hydrogen bonds and covalent bonds -- the removal of introns from RNA transcripts involves 2'-5' bonds made between parts of the strand).
2. RNA pairs adenine with uracil; DNA uses thymine, which is 5-methyl-uracil. The methyl group on thymine allows for more stable hydrogen bonding to make the characteristic double helix of DNA, while RNA is usually found single-stranded (with some exceptions). Also, the methyl group on thymine allows for a method of correcting errors in DNA sequence; there is an enzyme that reacts with 2'-deoxyuridine and replaces it with 2'-deoxythymidine in DNA.
Also, as January mentioned, it is thought that RNA may have been the start of life on this planet. It has been shown that RNA can spontaneously evolve from free nucleotides under the theoretical conditions of the planet 4 billion years ago. Given its ability to carry information as well as its ability to act as an enzyme (ribozyme), it seems a likely candidate.