What are core electrons and valence electrons?

(N/A) Core electrons: These are the electrons present in the inner,completely filled shells of an atom,corresponding to the nearest noble gas configuration.
Valence electrons: These are the electrons present in the outermost shell of an atom,which have the highest principal quantum number $(n)$. These electrons are involved in chemical bonding.
Example: For Sulfur $(Z = 16)$,the electronic configuration is $[Ne] 3s^{2} 3p^{4}$.
Here,the electrons in $[Ne]$ (i.e.,$1s^{2} 2s^{2} 2p^{6}$) are core electrons,and the $2 + 4 = 6$ electrons in the $3s$ and $3p$ orbitals are valence electrons.