An electrophilic aromatic substitution is an electrophilic substitution in which it is one or more hydrogen atoms on  an aromatic ring that are replaced by the electrophile.

eg. 1:

net reaction:

eg. 2:

net reaction:

see also arenium ion, Friedel-Crafts alkylation, Friedel-Crafts acylation, nucleophilic aromatic substitution