Purpose
The thesis statement:
Makes an argumentative assertion about a topic and states the conclusions that the author has reached about the topic
Makes a promise to the reader about the scope, purpose, and direction of your paper.
Identifies the relationships between the pieces of evidence that are being used to using to support your argument.
Is focused and specific enough to be "proven" within the boundaries of your paper.
See this definition and information about identifying a topic, deriving the main point from a topic, composing a draft thesis, refining and polishing a thesis, and completing the final thesis at the UW-M Writing Center.
"Developing a Thesis Statment." UW Madison Writing Center. University of Madison-Wisconsin. Web. 25 June 2010.
http://writing.wisc.edu/Handbook/thesis_def.html
"