A thesis is the idea put forth to be discussed or proven.
A dissertation is usually a written essay, often written by a student in order to get a higher degree (otherwise, it would probably just be called an essay). In order to get a doctorate you have to research and write so many pages on a subject.
So a dissertation will likely contain a thesis, and that up there in your quote is what's known as the "thesis statement", usually found near the beginning, where one clearly states exactly what the thesis is.
Now, dissertation and thesis can sometimes be used interchangeably, but usually that is only encountered pertaining to the writing assignment itself. So you can say that you "have to finish your thesis for class on Monday" instead of using "dissertation."