Class A refers to a very linear mode of operation for an amplifier, in which current is drawn from the supply continuously. It trades efficiency for low distortion.
MOS stands for metal oxide semiconductor.
This can refer to IC or discrete devices, generally FET (field effect transistor) based.
A network analyzer connects to a system under test with (generally) two ports, and characterizes each port for input and output properties over frequency. Measurements are often displayed in S parameter form, on Smith chart type graphs.