Which group is present in phenol? What is the effect of this group on the ring and its reactions?

(N/A) Phenol contains a hydroxyl $(-OH)$ group.
The $-OH$ group in phenol donates its lone pair of electrons on oxygen into the ring through resonance,making the ring electron-rich and activating it towards electrophilic substitution reactions. Thus,the $-OH$ group is an 'activating' group.
Due to resonance,the $-OH$ group increases electron density at the ortho and para positions. Consequently,electrophilic substitution reactions occur primarily at these $o, p$-positions. Therefore,the $-OH$ group is an 'ortho-para directing' group.
For example,electrophilic substitution reactions like nitration and bromination of phenol occur under mild conditions at the ortho and para positions. Due to the strong activating effect of the $-OH$ group,poly-substitution products like $2,4,6$-tribromophenol and $2,4,6$-trinitrophenol can be easily formed.