chapotraphouse

Despotism is an absolute monarchy where technically there's only one free man, the monarch, and all the subjects, including nobles or whatever, are his property. There's no contracts or something to this. The monarch's power is delegated as they wish to the state apparatus of whatever kind they choose. That'd be Russia since Peter or, say, Safavid Persia, and frankly a few German states in the new age despite sporting feudal symbolic.

Feudalism is a system of vassal-suzerain and allied relationships, contractual and not even that centralized, as a feudal entity can even be vassal to multiple other entities (games like CK get this wrong, which makes, say, Denmark there less fun than it was) and have its own various contractual ties. With feudalism every person's status is complex, there are individual privileges and obligations to every entity.

Feudalism is much more sophisticated than a despotic monarchy. IMHO it's the main reason Europeans dominate this planet.

EDIT: By the way, Decembrists paid a lot of attention to that difference, if we get back to Russian history.