A post-graduate diploma is often only a one-year course and is something that may not be connected with your first degree. They are often career related qualifications that just require a degree to get on to the course. For example, to become a teacher in the UK you can either do a first degree in teaching, or you can do a degree in say, history, then do a post-graduate diploma in education to qua
Diplomas are different to degrees. A diploma doesn't necessarily fall conveniently into this ladder of progression.
Degree - BA or BSc (sometimes called first degree but normally just degree). this is your first three or four years study at university (honours degrees are 4 years).
Masters Degree - the next level up. Normally 2 years, some 1 year, some 3 years.