RNA is present in several forms - messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). Each of these RNA forms is involved in different steps of protein synthesis. mRNA is generated from DNA and is the template for protein synthesis. tRNA is critical to the translation of the mRNA sequence into protein sequence. rRNA are components of the ribosomes which are the sites of protein synthesis.

RNA is a polymer that contains ribose rather than deoxyribose sugars. The normal base composition is adenine, uracil, guanine, and cytosine.

RNA can form double stranded duplexes. These duplexes are in the A conformation because the 2'OH
precludes the B conformation.

More commonly, RNA is single stranded and can form complex and unusual shapes. One example is tRNA which contains about 70 bases that are folded such that there are base paired stems and open loops. The overall shape of the completely folded tRNA is L shaped.

Another very interesting type of RNA is called a ribozyme, which is an RNA that has catalytic activity.
Several ribozyme structures have been characterized.

Ribosomal subunits have been studied in great detail [Gallery] | [Text]. The 30S ribosomal subunit and 50S ribosomal subunit consists of rRNA and multiple proteins.