When the electrophilic aromatic substitution takes place on a ring with substituents, the position of attachment of the electrophile is influenced by the electron-donating or withdrawing effect of the substituents.
The rate of reaction is influenced by the presence of substituents on the benzene ring and this fact goes in accordance with the role of the nucleophile of the aromatic ring. There are mainly two different effects:
Substituents that draw electron density away from the benzene ring tend to slow the reaction rate and they are addressed as deactivators. These favor a substitution in the meta position as pi acceptors.
Substituents that are electron rich and donate electron density to the benzene ring are called activators, and they promote substitutions in ortho or para positions as pi donors.
There are also cases where deactivators are sigma acceptors, in the case of highly electronegative atoms like halogens. These groups are deactivators, but they promote a substitution in ortho and para. Note that halogens are technically pi donors as well, but this effect is not noticeable when compared to their electronegativity.