What is the principle of IR spectroscopy/vibrational spectroscopy?

The principle of IR spectroscopy/vibrational spectroscopy is that molecules absorb specific frequencies of light that match the corresponding structure of the molecules. The energies are dependent on the vibronic coupling, mass corresponding to the atoms, and the shape of the molecular surfaces. An example of this is a molecule absorbing energy stored in incident light, causing a faster rotation. At temperatures above zero, atoms in molecules are continuously vibrating relative to each other. 

IR spectroscopy/vibrational can determine functional groups within a sample because different functional groups absorb different frequencies of IR radiation. The absorbing peaks of IR radiation are sharper compared to UV and visible regions.