Phenols is a group of molecules that contribute phenolic flavors and aromas.
Phenol is always present in beers, in the form of polyphenols such as tannins. Polyphenols are derivatives of hops and malt, and contribute to a dry mouthfeel.
Some (sometimes) wanted phenols are 4VG (clove, spicy, herbal), 4-EP (farmyard, medicinal, mice) and 4-ethylguaiacol (smoked meat, clove, spicy). These are introduced either through ingredients, process changes or addition of brettanomyces.
Peated malt contains a lot of phenols, contributing smoky, smoked meat character.