Classification ...

The amount of energy required to remove the most loosely bound electron from an isolated gaseous atom is called as first ionization energy $$(IE_1)$$. Similarly, the amount of energies required to knock out second, third etc. electrons from the isolated gaseous cation are called successive ionization energies and $$IE_3 > IE_2 > IE_1$$.
(i) Nuclear charge (ii) Atomic size (iii) Penetration effect of the electrons (iv) Shielding effect of the inner electrons and (v) electronic configurations (exactly half filled & completely filled configurations are considered extra stable) affect the ionization energies.
On the other hand, the amount of energy released when a neutral isolated gaseous atom accepts an extra electron to form gaseous anion is called electron affinity.
$$O(g)+e^-\xrightarrow {Exothermic} O^-(g); \Delta H_{eg}=-141 kJ mol^{-1}....(a)$$
$$O^-(g)+e^-\xrightarrow {Endothermic} O^{2-}(g); \Delta H_{eg}=+780 kJ mol^{-1} ....(b)$$
In (b) the energy has to be supplied for the addition of the second electron due to electrostatic repulsion between an anion and extra electron (same charged species). The electron affinity of an element depends upon (i) atomic size (ii) nuclear charge & (iii) electronic configuration. In general, ionization energy and electron affinity increase as the atomic radii decrease and nuclear charge increases across a period. In general, in a group, ionization energy and electron affinity decrease as the atomic size increases.
The members of the third period have some higher (e.g. $$S$$ and $$Cl$$) electron affinity values than the members of the second period (e.g. $$O$$ and $$F$$) because second-period elements have very small atomic size. Hence there is a tendency of electron-electron repulsion, which results in less evolution of energy in the formation of the corresponding anion.