1. Substituents already on the ring influence reactivity and orientation by interacting with the partial positive charges of the arenium ion intermediate. The groups are analyzed on the basis of resonance, hyperconjugation and inductive effects.

2. Activating ortho,para directing groups operate by stabilizing an adjacent positive charge through resonance (for groups with lone pairs on atoms attached to the ring) or hyperconjugation (for alkyl groups). These are sometimes called electron releasing groups. THE GOOD

3. Deactivating, meta directing groups operate by destabilizing an adjacent positive charge through an inductive effect. These are sometimes called electron withdrawing groups and they usually have pi bonds to heteroatoms on the atoms attached to the ring. THE BAD

4. Halogens are deactivators due to inductive effects, but ortho,para directors due to a resonance effect. THE UGLY.

5. On a practical level, you need to know where a new electrophile will go when reacting with an aromatic ring that already has a substituent. For monosubstituted benzenes, the answer is ortho/para when there is already a Good or Ugly group on the ring, but Meta when there is already a Bad group on the ring.