Veganism is a moral and ethical lifestyle choice with wide ranging implications whereas plant-based is a descriptor of the things that go into a foodstuff.

A vegan will only eat plant-based food (and so “plant-based” and “vegan” are often used interchangeably to describe a food) but someone who follows a plant-based diet might buy leather goods (and is therefore not vegan).

A good example of this nuance is the Impossible Burger. The Impossible Burger is plant-based but not vegan because it was tested on animals.