What is a simplified difference between Phase Shift Keying and Frequency Shift Keying?

FSK adjusts the frequency,
PSK adjusts the phase.
We use nonstandard PSK in our work because we can design a pre-amp and amp around the one frequency and design a filter with broad Q in the range of the expected signals we are trying to extract. We don’t bother with quadrature.
I am not sure if you are a beginner or not. If you are then forget about what I just said ;-)

Here is a good reference with pictures.
If you need more then look at fsk vs psk