The big two categories
For electrophilic aromatic substitution (EAS), every substituent already on the ring "directs" where the next electrophile goes. Activators donate electron density → ring reacts faster → product is ortho/para. Deactivators withdraw electron density → ring reacts slower → product is meta.
Why ortho/para for donors
Draw the resonance structures of the sigma complex (arenium ion) for ortho, meta, and para attack. When the donor sits in the right position to dump electron density into the positively charged ring, you get an extra resonance contributor that stabilizes the intermediate. That happens for ortho and para attack on a donor, but not meta.
Why meta for withdrawers
For a withdrawing group, the bad resonance structure puts the positive charge right next to the already-positive (or δ+) atom of the substituent — terrible. That bad structure occurs for ortho and para attack, ruling those out. Meta avoids it, so it wins by default.
Halogens — the exception
Halogens (Cl, Br, I) are deactivators (electronegative) but ortho/para directors (lone pairs can donate by resonance). The result: halogen-substituted benzenes react more slowly than benzene itself but still give predominantly ortho and para products.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.