In enhancement MOSFET channel is not initially exists but in depletion MOSFET channel is already exists.
In case of depletion MOSFET if the voltage is applied between drain and source the drain current will flow even at VGS=0. But in case of enhancement MOSFET, there is need to induce a channel for current to flow.
Depletion MOSFET can be operated in depletion mode and enhancement mode but enhancement MOSFET can only be operated in enhancement mode.
The default state of depletion MOSFET is on state means current flows without applying gate to source voltage, and the default state of enhancement MOSFET is off state means no current flows without applying gate to source voltage.
In depletion MOSFET voltage is applied to collapse the channel and in enhancement MOSFET voltage is applied to create a channel.