from Michael W. Pfaffl writing in Quantitative Real-time PCR in Applied Microbiology:
The present chapter describes the quantification strategies used in real-time RT-PCR (RT-qPCR), focusing on the main elements that are essential to fulfil the MIQE guidelines. The necessity of initial proper data adjustment and background correction is discussed to allow reliable quantification. The advantages and disadvantages of the absolute and relative quantification approaches are also described. In conjunction with relative quantification, the importance of an amplification efficiency correction is shown, and software tools that are available to calculate relative expression changes are presented.
Further reading: Quantitative Real-time PCR in Applied Microbiology Related publications