Detectable absorptions:

Not all vibrations can be detected through infra-red spectroscopy. IR energy is just only absorbed if the vibration directs to a change in the molecule's dipole moment. So, the symmetrical C=C stretching vibration of ethene does not effect in the absorption of IR energy, and no absorption peak is observed.  

Identification of functional groups:

IR spectra are specifically useful for identifying the existence of particular functional groups in a molecule, as the characteristic vibrations for these groups are known to take place in particular regions of the IR spectrum. For instance, absorptions because of the carbonyl stretching of an aldehyde take place in the region 1690-1740 cm^-1 while the corresponding absorptions for an ester take place in the region 1735-1750 cm^-1. IR tables can be employed to assign the several peaks and therefore the functional groups present.