Doing a PhD means undertaking original scientific research under the supervision of a professor. It takes on average 4 years. Many PhD candidates are employed by the University, but a number are subsidised by grants they have arranged themselves. The University also has a large number of external PhD candidates who generally work part-time on their research.
Leiden University offers opportunities for candidates to obtain their PhD, either directly with the university or via other organisations.
Also take a look under your faculty or study programme tab.