Structure Elucidation of Organic Molecules by IR Spectra:

For structure determination of organic molecules, the infrared spectrum could be widely divided within two regions. The region spanning from 3600 to 1200 cm ^-1 is known as the functional group region and the region which involves all frequencies below 1200 cm^-1 is called the fingerprint region. The two regions put together are significant in the determination of the identity of a molecule. It is almost not possible to assign all the possible frequencies observed in the IR spectrum. Therefore, identification of the features in the functional group region that involves stretching vibrations, of classically functional groups found in organic molecules, is quite significant.

It might not be probable to conclusively assign a signal to the functional group because a number of functional groups may have absorptions in a given frequency range. Therefore as the functional groups often give rise to a number of characteristic signals; we must look into other regions as well. Thus, the spectral interpretations should not be confined to one or two bands; the whole spectrum should be thoroughly examined. The features frequencies of some important functional groups are summarized in a correlation chart provided in Table. This would be quite useful to you while you attempt to interpret an unknown spectrum.

You can find a detailed account of how to use IR spectroscopy in the structure determination of organic compounds in Expt.4 of the MCHL^-3 course.  You would also learn how the IR spectrum can be used along with other spectroscopic tools in elucidating the structures of organic molecules.