Well, MOSFETs are of two kinds:
1. Depletion
2. Enhancement
In the Depletion type of MOSFETs, the channel between the Source and Drain already exsists. When a voltage is applied this channel gets depleted slowly. Hence the name Depletion MOSFET. So in essence, the conduction in this MOSFET will stop or come to a minimum after the entire channel has depleted.
In the Enhancement MOSFET, no channel exsists between the Source and Drain. When a voltage is applied, a channel is created between the source and drain thereby causing a current to flow between the terminals.
Enhancement MOSFETs are mostly used for normal ICs while depletion mode MOS are used for specific applications where we need this kind of behavior.