RNA polymerases, often abbreviated to RNAP, are the enzymes in a cell responsible for synthesizing strands of RNA. Because this function is essential to life, all organisms (and quite a few viruses) contain at least one type of RNA polymerase. Depending on the RNA polymerase, transcription requires either a DNA or an RNA template; although in most contexts, "RNA polymerase" refers to the DNA-dependant RNA polymerases, which use a DNA template to transcribe genes into mRNA ("messenger" RNA). Reverse transcriptases are enzymes (usually associated with retroviruses) that perform the opposite function, using an RNA template to synthesize a DNA product.
Most prokaryotes and archaeans use the same RNA polymerase to transcribe mRNA and functional non-coding RNAs (e.g. ribosomal RNAs or transfer RNAs). Whereas eukaryotes (organisms in which the genetic material is contained within a nucleus) use specialized RNAPs, each responsible for transcribing different classes of RNA.
RNA polymerase II (abbreviated RNAP II or RNA Pol II), is the enzyme responsible for transcribing the vast majority of protein-coding genes in eukaryotes. It is also responsible for transcribing the majority of genes encoding short nuclear RNAs (snRNAs) and micro RNAs (miRNAs). As such, RNAP II is the most well-studied and well understood of the eukaryotic RNAPs. Genes transcribed by RNAP II are referred to as "type II genes" and comprise the vast majority of genes present in eukaryotic genomes.
RNA polymerase I (RNAP I or RNA Pol I) transcribes most of the ribosomal RNAs in a eukaryotic cell (most of which are derived from a single transcript called the "large rRNA precursor").
Mitochondrial genes are transcribed by a separate mitochondrial RNA polymerase that is very similar in structure and function to bacterial RNAP.
1. Alberts et al. (2008). Molecular biology of the cell.