Basics:
Infra red (IR) spectroscopy deals with the interaction between a molecule and radiation from the IR region of the EM spectrum (IR region = 4000 - 400 cm-1). The cm-1 unit is the wave number scale and is given by 1 / (wavelength in cm).
IR radiation causes the excitation of the vibrations of covalent bonds within that molecule. These vibrations include the stretching and bending modes.
An IR spectrum show the energy absorptions as one 'scans' the IR region of the EM spectrum. As an example, the IR spectrum of butanal is shown below.
In general terms it is convienient to split an IR spectrum into two approximate regions:
4000-1000 cm-1 known as the functional group region, and
< 1000 cm-1 known as the fingerprint region