Generally in the US, a thesis is the final project for the masters degree and a dissertation leads to a doctoral degree. Those pursuing a masters degree must perform research on a specific subject that demonstrates their knowledge acquired through their program. Seeking a PhD is different in that your dissertation must contribute something completely new and undiscovered to your field. In other words, you have to contribute original knowledge to the subject. So the main difference between a thesis and a dissertation is the depth of knowledge you must attain in order to write the paper.