The thesis is the main idea of a piece of writing, its focus. Without a focus, either expressed or implied, an essay wanders and irritates and falls flat. With a focus, an essay is much more likely to click.
Here are some examples from the essays in The Bedford Reader :
These were two strong men, these oddly different generals [Ulysses S. Grant and Robert E. Lee], and they represented the strengths of two conflicting currents that, through them, had come into final collision.
- Bruce Catton, “Grant and Lee: A Study in Contrasts”
Inanimate objects are classified into three major categories – those that don’t work, those that break down and those that get lost.
- Russell Baker, “The Plot Against People”
It is possible to stop most drug addiction in the United States within a very short time. Simply make all drugs available and sell them at cost.
- Gore Vidal, “Drugs”
These diverse examples share a few important qualities:
The authors assert opinions, taking positions on their subjects. They do not merely state facts, as in “Grant and Lee both signed the document ending the Civil War” or “Grant and Lee were different men.”
Each thesis statement projects a single idea. The thesis may have parts (such as Baker’s three categories of objects), but the parts fit under a single umbrella idea.
Each thesis statement accurately forecasts the scope of its essay, neither taking on too much nor leaving out essential parts.
Each thesis statement hints about the writer’s purpose – we can tell that Catton and Baker want to explain, whereas Vidal wants mainly to persuade. (Explaining and persuading overlap a great deal; we’re talking here about the writer’s primary purpose.)