Quantitative PCR (Q-PCR or qPCR) is used to quantify starting amounts of nucleic acid template. In quantitative PCR, the DNA amounts can be measured either at the end of the PCR step (end-point assay) or during the PCR step (real-time PCR). Today real-time PCR is more commonly used because it can be more precise.
Frequently, quantitative PCR is used to measure RNA levels (although it can also be used to measure DNA levels). Isolated RNA is reverse transcribed into cDNA. Then quantitative PCR is run on the cDNA sample. This sub-technique is called quantitative reverse transcription PCR (qRT-PCR) or even longer names.