I. Topic vs. Thesis:
A topic is a general area of inquiry. Possible topics for an essay on Charles Dickens's novel Bleak House, for example, include "Police and the Law," "The Fallen Woman" and "First-v. Third-Person Narration." In stating the topic of an essay, a writer identifies his or her general subject matter. Stating a topic does not involve making a claim.
A thesis is a statement which makes a claim about a topic. In stating a thesis, a writer takes a stand on an issue. The thesis summarizes the writer's basic idea about a topic and lets the reader know what the essay is going to prove. The thesis of an essay about police and the law in Bleak House would make a claim about why law-courts and the police play such a prominent role in the novel. The thesis of an essay about narrative strategies in Bleak House would suggest a reason for Dickens's use of both first-and third-person narration in the novel.