A thesis is essentially the central argument you wish to convey in your paper. This tends to appear in the introductory paragraph, and the rest of the paper will consist of the supporting evidence. Having a thesis is the basis of a well-structured paper, since you will build the entire paper around proving that argument or analysis.
An outline is a rough rundown of the ideas, arguments and claims you plan on incorporating in your paper. Most outlines sketch out a tentative plan for the essay. For example, you would list the second paragraph as "B" and the three points you want to mention as "a," "b" and "c." Outlines also designate the importance or priority of each item and how you will make parallels between the items. If you need to include research or quotes, you can add them into the appropriate section in your outline.
Outlines and thesis statements are two ways of organizing your paper before you actually begin writing it. In performing these early preparations, you can make the process of writing run much more smoothly. Both outlines and thesis statements also allow you to test out your ideas and structure to make sure they are workable and supportable. A thesis helps you figure out what else you need to include in your paper, while an outline forces you to think about the structure and flow.
A thesis and an outline are not interchangeable. Both are essential for a well-crafted essay with a solid argument. A thesis is more focused on content. It demands critical-thinking skills so that you will come up with an assertion of your own that other people might disagree with. An outline, however, is more straightforward. As long as you have the research and the plans in place, you can put together an organized outline. You should write the thesis first, so that you can then arrange your outline around the central argument.