A thesis is just like a research paper in which the writer describes an argument also known as a thesis statement. For example, the writer's thesis topic is democracy and Pakistan. Therefore, his thesis statement is that democracy has not done good to the country. Once he has given this verdict, he needs to either prove or disprove the statement using relevant examples and research to assist his findings. The reason of a thesis is to display a student's knowledge, analytical thinking and understanding of the subject he is enrolled in. As a result, the student's purpose is not to make new theories but showing his understanding of the field. Make sure you know that a thesis gives analysis to preexisting literature.
On the other hand, a dissertation is a paper that furthers a writer's viewpoint about a subject that he is studying like literature. The writer's point of view is influenced by a lot of research but is dependent on the writer's thoughts and research. In simple words, it is based on a theory or hypothesis a writer is supporting or against. For instance, a dissertation hypothesis is that women are bad drivers. In this he needs to list original research findings like surveys, statistics and interviews which agree or disagree with the hypothesis. In a dissertation you have to add new and unknown detail to the field that you are studying which is you are contributing and innovating at the same time. Therefore, understand that a dissertation should include analysis of original literature in the field of study.