Esters are fruity fermentation byproducts — banana, pear, apple, stone fruit are common tasting-note descriptions — produced by yeast as a natural part of fermentation, with the amount and character varying enormously by strain and, for a given strain, by fermentation temperature.
The same chemistry, wanted in one style and unwanted in another
Warmer fermentation generally pushes ester production higher, and certain yeast strains are simply far more ester-forward than others regardless of temperature — Belgian and German wheat-beer strains are deliberately chosen for pronounced ester character, since it's a core, desired part of those styles' identity. Clean American ale strains are bred and selected specifically to minimize ester production, because the same fruity character that defines a Belgian Blonde Ale would read as a flaw in an American IPA chasing clean, hop-forward clarity.
Esters and phenolics are a genuine pair, not the same thing
Esters (fruity) and phenolics (spicy, clove-like, sometimes smoky or medicinal) are two chemically distinct classes of yeast byproduct that often get discussed together, particularly for Belgian and German wheat styles where both are present at once — Weissbier's classic banana-and-clove profile is literally one ester compound and one phenolic compound, produced by the same yeast strain, working together to create the style's signature flavor.