ionisation energy is the energy requered to remove an electron from an isolated gaseous atom in it''s ground state while work function is the minimum energy that must be possessed by a photon to eject electrons from the surface of metal on which it is incident,in other words it is the minimum energy that must be transferred to an elctron to eject it from the metal surface.

Ionisation Energy:

The minimum energy required to remove the electrons from the outermost shell of an atom.

Work Function:

A certain minimum amount of energy is required to be given to an electron to pull it out from the surface of the metal.This minimum energy required by an electron to escape from the metal surface is called as the work function of the metal surface.

ionization energy refers to energy supplied in form of heat to excite one or two or at most four electron and remove them from the isolated atom ONE BY ONE,
where as work function refers to a particular amount of energy in terms of PHOTONS which when strke the metal surface help in excitation and removal of electrons ALL AT ONCE.

Ionization energy is the minimum amount of energy required to remove an electron from the outermost shell of an atom, thus giving it a charge.

Work function is the minimum amount of energy required to remove an electron from a solid surface to just outside of the surface(as in photoelectric effect). The final electron position is far from the surface on the atomic scale, but still too close to the solid to be influenced by ambient electrostatic potentials in the vacuum.