Depletion MOSFET is a MOSFET in which the channel already exists. If we apply a voltage between drain and source then the drain current will flow even if the gate to source voltage is zero. Depletion MOSFET can be operated in depletion mode and also in enhancement mode.
The portion between drain and source is diffused with the same impurity which is used for drain and source and it forms a channel between drain and source then it is called depletion MOSFET as shown in figure. The drain current will be reduced as the gate to source voltage made more negative.