Diploma programmes have two key purposes. If the level of your previous qualification or its subject content does not permit progression directly to a master's, you can use a diploma programme as a conversion or refresher course. A diploma is also a qualification in its own right, which extends the range and depth of your undergraduate studies.
Diploma students attend specially arranged classes and tutorials; in most, but not all, cases you would attend lectures from undergraduate courses. Most diplomas are assessed by formal written examinations, though a number have assessed work attached to certain courses. Some LSE diplomas are structured as the first year of a two-year MSc, such as those in the Economics Department; others exist as alternative exit points for modular executive MSc programmes.