What are enhancement and depletion type MOSFET?

Enhancement-mode transistor: Normally OFF

They develop a conducting channel between the source and drain only when a voltage is applied at the gate and source terminal (Vgs).

i.e. the channel does not exist when Vgs = 0V. If a positive bias voltage (+Vgs) is applied at the gate, then a channel is formed and drain current starts to flow through the channel. 

Advantage:- Prevents false triggering of the circuit.

Depletion-mode transistor: Normally ON

Have a conducting channel between the source and drain which makes them inherently ON without applying any control voltage at the gate.

i.e. the channel is fully conductive and current flows strongly between the drain and source when the gate voltage is at 0 volts (Vgs = 0V). For shutting down the current carriers in the channel, apply a more negative gate voltage than the threshold voltage i.e. (– Vgs – Vth).