Both are techniques used to analyse a sample by the measurement or observation of radiation either passing through or being emitted by the sample. The process produces a spectrum, a graph of intensity versus a property of the radiation, typically the wavelength or frequency of EM radiation. ("Spectra" is the plural of "spectrum".)
Spectroscopy or spectrometry?
If you use EM radiation (e.g. visible light) to acquire a spectrum, we call it spectroscopy.
If instead you are measuring particles in some way to give a spectrum, we call it spectrometry.
Hence IR spectroscopy and ¹³C NMR spectroscopy, but mass spectrometry.
Absorption vs emission
In emission spectroscopy, the sample is provided with energy, or excited, and the resulting emission of EM radiation is analysed.
In absorption spectroscopy, EM radiation is passed through the sample and the remaining light is analysed and compared to a blank (no sample) spectrum.
Absorption spectroscopy is the focus of this topic.
Which part of the EM spectrum does what
Radiation
Frequency (Hz)
What it probes
Radio frequency
10⁶–10⁹
Spin transitions of charged particles: the basis of NMR
Microwaves
10⁹–10¹¹
Rotational spectroscopy: size and rotational nature of the molecule