While expository and persuasive works both use a thesis statement and research, the difference between them is that an expository essay explains while a persuasive essay argues. In other words, the expository writer provides clarifying information about a topic -- asking "What?" -- while the persuasive writer presents an argument, asking "What if?"
Expository Essays Explain
An expository essay is an investigative document that requires a student to research and evaluate information about an idea; its thesis is a focused statement that the student's evidence clarifies. An expository essay explains, for example, how the events of Pearl Harbor devastated a nation.
Persuasive Essays Debate
A persuasive or argumentative essay's thesis is a debatable point; the essay presents evidence not to explain but to support an argument or advocate a particular viewpoint. A persuasive paper might argue, "Pearl Harbor was a catastrophe that President Roosevelt could have averted."
These Essays Can Swap Structures
Persuasive essays can incorporate expository sections when you need to explain yourself before laying out an argument. Expository essays can present an argument that they are explaining. A persuasive essay might examine creationism versus evolution; an expository essay would explain the debate in light of the Scopes trial.