What is Carbon-13 NMR?

Carbon-13 NMR records the resonances of the $\ce{^{13}C}$ nuclei of a sample. It is usually run with broadband decoupling: the protons are irradiated over their whole range of frequencies during the measurement, which removes every carbon–proton coupling, so that each kind of carbon gives one single line. Equivalent carbons, exchanged by a symmetry of the molecule (or by a fast rotation), give the same line.