There is always depletion region when Vg > VFB.
At Vg> Vt though, the depletion regions is "screened" (or shielded) by the inversion layer (a thin layer of electrons in the channel, at the Si/oxide interface, supplied from N+ source/drain regions).

If you want to understand what all these terms mean - depletion, deep depletion, accumulation, inversion - you can read any textbook on semiconductor device physics (for example, S.Sze book "Physics of semiconductor devices").

Max