A plant based diet is commonly mistaken for veganism because the two have some similarities. Yes, they both are heavy in receiving nutrition from plants. However, there is a clear difference between the two.
Vegans eliminate ALL animal products from their diet, AND from all aspects of their lives. We’re talking – no leather, fur, wool or silk; products derived from insects (i.e. honey and beeswax); or toiletries that may be derived from and/or tested on animals.
Plant based- may consume only plant-based products but wear/use products that are derived from animals. (allowed honey)
Veganism is a lifestyle. When people commit to this, it isn't solely what they eat. It's everything that they associate with. When you "go vegan" it isn't just about not eating meat but still owning that really nice pair of leather shoes. Another BIG factor that goes into being vegan is knowing the manufacturing conditions. A vegan does not use brands that are not animal cruelty free. Also, they pay attention to what is added to products because a lot of things that may not even be considered a threat, such as candy, will contain some type of animal byproduct. Therefore it is VITAL that a vegan read the labels of practically everything.
The plant based diet is more lax in the sense that there aren't many rules. People who maintain this diet eat a lot of plant foods. They tend to have 50% raw veggies and 50% of cooked plant foods. These people also don't use dairy or animal byproduct in food but they still may use honey, as mentioned before. The biggest difference between the two is that, it isn't a moral issue for a person of a plant based diet to use a cosmetic product containing an ingredient derived from oil produced from an animal. Most people who eat a plant based diet, although yes they may care passionately about animals, do it because the body functions more efficiently for them doing so.
The main difference between the two is basically, veganism is a lifestyle choice while a plant based diet is just that, a diet full of plants!