To the best of my knowledge (what may not be ultimately correct):
Calibration is to reduce bias, or to make the measured value (better) truely match the quantity of the analyte. In qrtPCR, external standards of a fixed (and possibly known) concentration can be used for calibration, making measurements to agree with that standard (or even with a known concentration of the target sequence, if this is known for the standard used). A typical application is to record so-called calibration curves from dilution serias of such a standard. For instance, you can use a plate-calibrator to make Ct values comparable across different plates.
Normalizer can be anything you normalize a value to. It's use in qrtPCR is to eliminate stocastic variability between samples / sample preparations (specimen, sample materials), and thus to make (possibly calibrated!) measurements comparable across samples. In contrast to calibration, normalization is used to get the measured values from different samples on a "common ground". It is often not possible to control the amount of sample that goes into a qrtPCR, so that the measured concentrations may not be directly comparable, because the amount of sample to begin with was already different. Normalization to some measurable quantity that correcponds to some meaningful biological unit in the different samples allows to compare these "relative quantities". This measureable quantity could be anything, and in qrtPCR it is convenient to use the concentration of a different sequence which concentration is representative for the number of cells that are included in the sample (aka "reference gene").
There is some overlap in the meaning, since the elimination of between-plate/run variation is in principle and the elimination of between-sample variation are both to make measurements comparable. Calibration is based on an external, fixed or defined standard (of which the concentration may or may not be known), whereas normalization is based on a measured quantity of the sample itself.
In the literature, these concepts and terms are often mixed and used wildly and wrongly. So, in practice, you may use any term for anything and nobody will have any concerns.
Dear Basma,
Jochen Wilhelm has explained it nicely. To the best of my knowlwdge and to be very specific, Calibrator is something that is required to adjust the machines sensitivity towards every dye. Calibration is done for every machine to maintain and adjust the sensitivity at optimum level.
Normalization is done for each samples for each reactions to negate the human error during pipetting of the samples or rather during sample preparation.
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