ESR (electron spin resonance) is also known as electron paramagnetic resonance (EPR) spectroscopy. It is a technique which can be applied; at least in principle to all systems where unpaired electron spins are present. for example it allows the determination of structural properties on a molecular scale; the selective characterisation of redox-active centres in proteins; the study of the kinetics of chemical reactions.
For details it would be best to check either phys chem textbooks eg. Atkins or prehaps see if there is a book in the oxford chemistry primers series which covers the topic in greater depth.