The Thesis or the Research Question is what a text is about while the Title reflects the most engaging, concise expression of that focus.
In many disciplines the terms thesis and research question are used fairly interchangeably. That said, thesis is more commonly used in popular discourse and academic writing whereas research question is used in in the social sciences and sciences to focus empirical research.
Some people consider purpose to be synonymous with thesis, yet more commonly purpose is used to classify texts at a broader, more global level (e.g., expository discourse vs persuasive discourse).
To communicate with others, writers need to focus on one a dominant thesis or research question. A focus on a single topic empowers a writer to create Unity, Coherence, Flow and to develop substantive content.
To help you develop an effective thesis or research question, see