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 "real-time PCR". This is a PCR process that requires highly sensitive equipment for measuring fluorescence throughout the PCR process.
In real-time PCR, the PCR mixture contains a reagent that causes a fluorescent signal in proportion to the number of newly-formed DNA strands. The PCR reaction is run in a 96-well plate and the thermocycler is equipped with a very sensitive camera that can measure fluorescent signals and quantify the amount of DNA formed. Different techniques exist, based on the use of different fluorogenic reagents. Two of these are the probe-based and intercalator-based methods.
An advantage to RT-PCR, over conventional PCR, is that quantification can take place much sooner, without waiting until completion of the entire reaction and using gel electrophoresis. The sensitivity for detection is much greater than agarose gels, and can differentiate between as little as a 2-fold change in DNA concentrations, versus 5-fold-plus changes detected by electrophoresis methods. Another advantage is that quantification takes place before the PCR products begin to degrade, or the reaction rates plateau.