What are the three main types of RNA? What is meant by heterogeneous RNA?

Messenger RNA, or mRNA, transfer RNA, or tRNA, and ribosomal RNA, or rRNA, are the three main kinds of RNA.

The newly produced RNA molecule, a precursor of mRNA, is called heterogeneous RNA (hnRNA). The heterogenous RNA bears portions known as introns and portions called exons. The hnRNA is processed in many chemical steps, introns are removed and mRNA is created formed only of exons, the biologically active nucleotide sequences.