Spectrum from McGrawHill <http://www.mhhe.com/physsci/chemistry/carey/student/olc/ch13ir.html>
Reading an IR spectrum may seem incredibly difficult at first due to the complex looking graphic, but it is not. First of all, the very complex looking region below 1500 wavenumbers is called the fingerprint region and can mostly be ignored. The fingerprint region contains bond stretches between backbone atoms and is usually too much of a mess to be any help. Instead, concentrate on the region between 1500-4000 wavenumbers. This region contains peaks that correspond to stretches in functional groups, and is all that is usually needed to determine molecular structure. The chart below lists many of the most common frequencies (in wavenumbers) of peaks seen in a spectrum and what functional group they correspond to. By finding the large peaks in the functional group region and seeing what functional group bond stretch they correspond to, a model of the molecule can begin to take shape. The depth of the peak correlates with how much of the substance is present in the sample. It is helpful to have other data from other instrumentation, such as chemical formula (mass spec) or number of carbons/hydrogens (NMR), to confirm a proposed molecular model.
Table 1 Table showing most common IR frequencies and bond stretches.