Quantum Chemistry/Multivariable differentiation

Provide a brief description of partial differentiation.

Example

Show the calculation of the derivative of this wavefunction with respect to x and y for the function $\psi (x,y)=A\sin \left({\frac {n_{x}\pi }{L}}\right)\cdot \sin \left({\frac {n_{y}\pi }{L}}\right)$