Postgraduate Certificates (PGCert) tend to cover vocational subjects and are ideal if you want to expand on existing knowledge for career purposes.
Fairly short in length, most PGCerts take around 15 weeks to complete.
The main exception to this is postgraduate certificates in education (PGCE), which take around a year. Taught just like an undergraduate degree, you'll be graded with a pass, merit or distinction.
Most PGCerts can be converted to a master's degree, and some can be used to advance existing qualifications. For example, you could study for a Postgraduate Certificate in Medical Education once you're already a doctor.