IR and proton NMR spectra: identifying a molecule

A bond absorbs the IR radiation whose frequency matches its vibration. The spectrum plots transmittance (%) against wavenumber, from $4000$ to $400\ \text{cm}^{-1}$: an absorption is a dip. A band points to a bond, hence a functional group. Below $1500\ \text{cm}^{-1}$: the "fingerprint", unique to each molecule, which is not analysed band by band.