The conjugated [tex] \pi [/tex]-system in the aromatic ring produces the effect of a ring current when the magnetic field from the NMR spectrometer is applied. This ring current in turn induces its own magnetic field which coils around the C-C bonds (right hand rule) and opposes the applied field within the ring, and is parallel to it outside of the ring. Due to this, the aromatic protons 'feel' an even stronger magnetic field than is applied, and require higher energy and therefore higher frequency radiation to resonate, shifting them downfield from regular aliphatic protons.
