RT-PCR is a somewhat confusing term because it can refer to two different methodologies used in biotechnology and genetic research. One of the definitions is "reverse transcriptase PCR". This is a PCR process that uses reverse transcriptase enzymes to make DNA from an RNA template. Reverse transcriptase, or RNA-dependent DNA polymerase, is a type of enzyme commonly found in retroviruses.
The PCR mixture that is used in RT-PCR is very similar to that used for normal PCR, including the DNA polymerase enzyme. The difference is the added reverse transcriptase enzyme is also added. This enzyme synthesizes single-stranded DNA. DNA made using an RNA template is called cDNA. The cDNA then undergoes "normal" PCR processes, under the control of DNA polymerases, resulting in the synthesis of millions of double-stranded DNA molecules representing the sequence of the original RNA.
RT-PCR is used to detect and quantify messenger RNA (mRNA). It is an important tool because only a small fraction of RNA in the cell is mRNA, and mRNA is destroyed after translation, so it never accumulates to very high concentrations. RT-PCR is used in proteomics to make cDNA libraries, which can aid the study of gene expression and sequences.