Fundamental absorptions are vibrational transitions from the ground state to the first excited state. They give rise to the bands of greater intensity.
Overtones are vibrational transitions from the ground state to higher excited states. The absorption frequencies are $2/nu,3\nu...$ where $\nu$ is the frequency of the fundamental absorption.
Combination bands, result from the coupling of 2 bands $\nu_1$ and $\nu_2$ to give rise to a new frequency band $\nu_{comb}=\nu_1 + \nu_2$
Difference bands, , result from the coupling of 2 bands $\nu_1$ and $\nu_2$ to give rise to a new frequency band $\nu_{comb}=\nu_1 - \nu_2$
Fermi resonance, result from the coupling of a fundamental absorption band with an overtone or a combination band.
