Mass spectrometry (MS) makes use of the motion of ions in electric and magnetic fields in order to sort them according to their mass-to-charge ratios. Thus, MS is an analytic technique by which chemical substances are identified by the sorting of gaseous ions in electric and magnetic fields. The instruments used in these studies operate on the principle that moving ions may be deflected by electric and magnetic fields. A device that performs this operation and uses electrical means to detect the sorted ions is called a mass spectrometer.
To answer this posting I will have to tell you what mass spectrometry is in laymans terms first.
Mass spectrometry is a technique where one can analyze the molecular mass of a compound directly. Say you have made a compound X and wanted to be sure that it was what you thought it was, you could analyze the molecular mass of the compound, compare it with the molecular mass that you believe it should be (adding up the elemental masses from the periodic table) and identifying (characterizing) your compound.
There are several different types of MS that are discussed, and ...
The solution discusses what is mass spectrometry in 425 words in the solution.