The terms dissertation and thesis are more-or-less interchangeable. Usage differs from country to country and occasionally from institution to institution. In Britain, dissertation is more likely to refer to Master's level works, and thesis to PhD or other doctoral works. In other countries, the reverse may be true.
This guide uses the term dissertation to refer to all dissertations and theses.
Doctoral dissertations are more likely to be available online. Master's dissertations are not even always kept in hard copy by the host institution.