Common examples of essentialism are racism and sexism.
See also Logical fallacy/No true Scotsman
In more philosophical terms essentialism's roots go back to Plato who placed the idea at the center of each thing.
According to essentialism the idea "behind" (better: contained in ) each thing directs its development.
Critics like modern evolution biologists or german philopsopher Karl Popper strongly dispute the existence of inherent ideas and refer them back to the human mind where they originated. Instead they focus on interaction of an entities' features with features of its environment in order to explain an entities' development.
See: structuralism Constrast with: constructionism, poststructuralism