Reactivity of phenol:

The lone pairs on the oxygen are donated to the electron cloud. The cloud is now more electron dense. This allows the ring to induce a dipole across halogens. The positive end of the halogen induced dipole acts as an electrophile.

Activating groups

EDGs are known as activating groups. They donate electrons.
NH2 is an activating group. It allows the aromatic ring to react readily with electrophiles. So the side groups go to 2,4 and 6. NH2 and OH are activating.
NH2 is 2,4 directing.

Deactivating groups

EWGs are known as deactivating groups. They attract electrons.
Deactivating groups withdraw electrons from the cloud. This decreases electron density. The side groups go to 3 and 5. An example of a deactivating group is NO2.
NO2 is 3 directing.

Reactions of benzene - activating and deactivating groups