basic rna structure

Most cellular RNA molecules are single stranded.  They may form secondary structures such as stem-loop and hairpin.

Figure 3-C-1.  Secondary structure of RNA.  (a) stem-loop.  (b) hairpin.

The major role of RNA is to participate in protein synthesis, which requires three classes of RNA:

messenger RNA (mRNA)
transfer RNA (tRNA)
ribosomal RNA (rRNA)

Other classes of RNA include

Ribozymes
The RNA molecules with catalytic activity.
Small RNA molecules
RNA interference and other functions.