You know what? I always used to think protists are also plants until recently that I came across the difference online. You might be like me too. But plants belong to a kingdom, while protists belong to different ones. In pants cells, the nuclear DNA strands have higher complexity compared to those in protists. In addition, all plants are multi-cellular, which is unlike protists.
Conversely, some of the protists are unicellular, while even some others live in colonies, performing some tasks together, such as movement and feeding. And still, you need to be informed that even when some of these protists are unicellular, some others are multi-cellular; a typical example is a seaweed.
As though in science, plants are referred to as autotrophs or primary producers, that means they produce food through inorganic molecules. But in the case of protists, while some are autotrophs, such as algae, others are heterotrophs or consumers.