Quantitative PCR (Q-PCR or qPCR) is a PCR technique used to quantify starting amounts of DNA template. Amounts can be measured either at the end of the PCR (end-point assay) or during the PCR steps (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 instead of DNA levels. To do this, isolated RNA is reverse transcribed into cDNA. Then quantitative PCR is run on the cDNA sample. This sub-technique is called quantitative reverse transcriptase PCR (qRT-PCR) or even longer names.