Hi, here is a brief summary. Let me know if your would like further elaboration.
1) 1H/13C NMR - detecting the connectivity of molecules (ie the exact structure), one of the most useful spectroscopic techniques
NMR involves irradiating molecules with radio wave frequencies while they are exposed to extremely strong magnetic fields. Each nucleus is in a unique chemical environment due to different amounts of electron density surrounding it, thus resulting in absorption at different radio wave frequencies.
2) IR Spectroscopy - determines functional groups of molecules only
IR Spectroscopy involves irradiating molecules with IR radiation and observing what frequencies of IR are absorbed. IR absorption corresponds to molecular motion (ie stretching of a C=O bond), and so the absorption of IR frequencies corresponds to the identity of a particular functional group. IR gives you less info than NMR so it's rarely used today for confirmation of structure.
3) Mass Spectrometry - gives you molecular weight and sometimes formula + limited information on connectivity.
There are many types of mass spectrometry, the on the MCAT is more commonly known as electron impact mass spectrometry. A molecule is bombarded with electrons which knocks one electron out of the molecule, resulting in a positively charged molecular ion. This ion is accelerated through a magnetic field, potentially fragmenting in the process, finally hitting a detector and having its mass recorded. The magnetic field strength is changed to scan through different masses of ions, giving a histogram of relative frequencies that different ions hit the detector.
4) UV Spectroscopy - usually for protein/DNA quantification, doesn't really tell you much about structure/connectivity
Some highly conjugated molecules absorb UV light due to a small gap between their highest filled and unfilled molecular orbitals. This technique is mostly used to quantify proteins containing tryptophan, as tryptophan absorbs UV light.