A dissertation is a paper that advances an author's point of view about a topic he's studying, such as English. The author's point of view is guided by research but is based on the author's thoughts and original research. In other words, a dissertation is based on a hypothesis or theory an author is trying to prove or disprove. An example of a dissertation hypothesis is that texting revolutionizes and improves grammar. However, unlike a thesis, a dissertation is not trying to prove the point by quoting someone else's research. Instead the author must provide original research findings --- such as surveys, interviews and statistics --- that prove or disprove the dissertation theory.
A thesis is similar to a research paper. An author introduces an argument, called a thesis statement. For example, an author's thesis topic is texting and the English language. Therefore, the author's thesis statement is that texting ruins grammar. Once the author presents that statement, he must either prove or disprove the argument, using research to support his claims.
The purpose of a thesis is to show a student's level of understanding, knowledge and analytical thinking in the field he's studying. Therefore, a student completing a thesis is not concerned with making new discoveries but demonstrating his knowledge of the field. A dissertation requires a student to contribute completely new and undiscovered information to the field that he's studying, according to Campus Explorer (campusexplorer.com). That is, he is contributing something original on the subject. For example, a student writing a dissertation is trying to discover something, such as a way that texting improves grammar, rather than rehashing information that another person has discovered. In other words, the best way to remember the difference between a dissertation and a thesis, according to the Difference Between (differencebetween.com), is that a thesis adds analysis to already existing literature, whereas a dissertation --- which may include analysis of existing literature --- is intended as an addition to the body of original knowledge in a field of study.
Although degree requirements vary from university to university, a dissertation and thesis are parts of different programs. For example, a thesis is typically for a graduate program leading to a master's degree. A dissertation, however, is usually a part of a graduate program leading to a doctoral degree, or Ph.D.