A VASP simulation requires four files initially to do a simulation: INCAR, POTCAR, POSCAR, and KPOINTS.
The INCAR file controls the options of the DFT simulation through a series of tags. The number of tags required for a simulation is dependent upon the complexity of the simulation.
The POTCAR file is the psuedopotential file that describes election energy exchange correlation functional.
The POSCAR file is a structure file which describes a material through a representative volume elements bounded by periodic boundary conditions and atoms contained within the representative volume element.
The KPOINTS file is a file which describes the points in the reciprocal lattice from which the Kohn-Sham equations are numerically estimated