Aromaticity and electrophilic aromatic substitution

A system is aromatic if it is cyclic, planar, fully conjugated (one p orbital on every ring atom) and holds $4n + 2$ π electrons, $n$ being an integer: $2$, $6$, $10$… With $4n$ electrons in the same geometry it is antiaromatic, hence destabilised: cyclobutadiene ($4$ π electrons). If planarity or conjugation is missing, it is simply non-aromatic.