Students reaching at the end of their academic years are usually asked to submit a unique piece or work which illustrates their knowledge and will to look for knowledge along with the demonstration of expertise necessary to conceive such a piece. It can be a thesis or a dissertation.
In USA “thesis” is needed for either diploma, undergraduate or Masters degree, whereas “dissertation” is required to be written at PhD level.
In UK, “dissertation” is needed for either diploma, undergraduate or Masters degree, whereas “thesis” is needed for a degree award of MPhil/PhD.
A thesis is much shorter usually ranging up to 100 pages which is subject to change at the will of the University/Institution or the academic requirements.
Being more detailed that a thesis a dissertation ranges from 250-300 pages which is subject to change at the will of the University/Institution or the academic
In a thesis the information used is the information which has been collected by someone already. The data composed in a thesis is based on the hypothetical analysis of contents
In a dissertation you have to conduct a thorough research and come up with information of your own. A dissertation is composed of theory and argumentation based on original research.
A thesis being shorter in length and requiring less amount of research and study requires considerably less amount of time and usually students are tasked with a thesis when less time is available.
A dissertation being the longer and detailed version with all the research and writing process is what requires tremendous amount of time and this is why it is tasked when there is sufficient time available.
In a thesis you have an objective and a point to prove and all your efforts are directed towards proving it failing which one can fail the thesis itself.
In a dissertation you have to focus on everything and in some instances if you are unable to prove everything about your research proposal you have to accept and defend the limitations of your research process.
A thesis is more like an extended term paper or a research paper. A thesis is the mere hypothetical analysis of the data available already.
A dissertation is more like a book and its usually more publish worthy too. A dissertation is the combination of original research and established theories.