Acronym for Metal on Oxide Semiconductor - Field Effect Transistor.
An nMOSFET is a device made by defining doping areas in a silicon wafer. Using a p-doped substrate(silicon wafer) we dope (using diffusion) two blocks separated by a channel of p-doped substrate. The channel area between the two n-doped regions is exposed to water/air to form an oxide layer (Si02 - commonly known as glass). This oxide layer is used as insulation and a polysilicon (silicon in a non-uniform lattice structure) gate is deposited on top of the oxide layer.
When no power is applied to the gate the device acts like two back-to-back diodes, letting no current flow. When a positive voltage is applied to the gate, electrons are attracted to the channel region, forming a (temporary) n-channel region between the two n-doped terminals. This allows current to flow.
A pMOSFET is the complete opposite of an nMOSFET, the terminals are p-doped, the substrace is n-doped and the channel is formed when the gate is connected a negative voltage (with respect the highest voltage terminal), in most cases GND is negative compared to the highest voltage terminal.
CMOS is a process (and a LogicFamily) that refers to the creation of digital gates using a combination of pMOSFET and nMOSFETs in a way that they compliment each other, allowing for 0A steady state current draw, fast switching speeds, high noise immunity and low size.