What is NMR spectrum, chemical shift?

A proton NMR spectrum (nuclear magnetic resonance) shows the signals absorbed by the hydrogen nuclei of a compound. The position of a signal is its chemical shift $\delta$, in parts per million ($\mathrm{ppm}$), measured from the signal of a reference compound, tetramethylsilane $\ce{Si(CH3)4}$ (TMS), set at $\delta = 0$. The $\delta$ axis is drawn increasing from right to left.