It's probably going to depend on the field and the level Daria. In the UK we have undergraduate diplomas and postgraduate diplomas and both undergraduate and post graduate certificates. Some also have their own names and qualify you for certain things, and some are accredited by professional organisations rather than 'academic conventions'.
A PGCE (Post Graduate Certificate in Education) is a certificate taken by a Graduate to qualify them as a teacher, for example. A P.Dip.Ed would be one level up - a Post Graduate Diploma in Education.
What area are you talking about and I might be able to point you to the UK equivalent?