Absorption bands in the range of 4000 - 1500 wavenumbers are typically due to functional groups (e.g. -OH, C=O, N-H, CH3, etc.). The region between 1500 - 400 wavenumbers is referred to as the fingerprint region. Absorption bands in this region are generally due to intra-molecular phenomena, and are highly specific for each material. The specificity of these bands allow computerized data searches to be performed against reference libraries to identify a material.

Applications
Material Identification - The unknown IR absorption spectrum is compared with standard spectra in computer databases or a spectrum obtained from a know material to determine the identity of the material being analyzed. Matches identify the polymer or other organic constituent(s) in the sample.

Quantification - Quantitative concentration of a compound can be determined from the area under the curve in characteristic regions of the IR spectrum. Concentration calibration is obtained by establishing a standard curve from spectra for samples of known concentrations

(Source: http://www.mee-inc.com/ftir.html)