Upon completion of their university education, undergraduates earn the title of Doctorandus (abbreviated rather confusingly as `drs.'). This signifies `he/ she who is preparing to defend a thesis' and it earns one the right of preparing a Ph.D. thesis (proefschrift). The Doctorandus title is roughly equivalent to the Master's degree in the USA or UK. Note that undergraduates are referred to as studenten, which translates literally as students. This may cause some confusion between undergraduates and Ph.D students. Simply remember that when you hear the Dutch person discussing students, they are referring to undergraduates.
Recently, the Dutch universities have started to implement an inter-European agreement, known as the Bologna Agreement, to restructure university education along roughly Anglo-American lines, with a three-year bachelor and a two-year master phase. Thus, a bachelor will be a bachelor, but a master will be a doctorandus as well as a master. Note: graduates from one of the Technical Universities (polytechnical schools) are usually Ingenieur - ir. - (Engineer) instead of Doctorandus - drs. - whereas graduates from a University Law School have been Meester - mr. - (Master) already for a long time.
Ph.D students are referred to as promovendi. This stems from the fact that when you are awarded a Ph.D degree you are `promoted' from Doctorandus to Doctor. The Dutch word promotie refers to the actual Ph.D examination (thesis defense) which is a very formal ceremonial happening of exactly one hour duration taking place in the venerable Academie Gebouw on the Rapenburg. Although the questioning may be tough, the candidate never fails ...
Now prepare for the more complicated part of the ranking system in Dutch universities. The permanent scientific staff consists of Universitaire Docenten (UD), Universitaire Hoofddocenten (UHD), and Hoogleraren (Professors). The UD is sort of equivalent to a University lecturer and the UHD is equivalent to an associate professor. The peculiar thing about the Dutch university system is that only Hoogleraren are allowed to award Ph.D degrees to promovendi. This means that formally only a Hoogleraar can be a thesis advisor (in which case he or she is referred to as the promotor of a particular promovendus). However, in practice both the UD and UHD can be the supervisor of a Ph.D student, and both can act as co-promotor. In such a case one of the Hoogleraren is formally the promotor and acts as such during the thesis defense.
The story does not end here. There are different kinds of Hoogleraren at Dutch universities. There is the `gewone' (normal) Hoogleraar, the Buitengewoon Hoogleraar and the Bijzonder Hoogleraar. The first category is the usual full professor, with a fulltime university position. The Buitengewoon Hoogleraar is someone with a job somewhere else who is appointed as part-time professor. The Bijzonder Hoogleraar is an adjunct professor who also has a position elsewhere together with an honorary professorship in Leiden. The Buitengewoon Hoogleraar is funded by the university. The Bijzonder Hoogleraar is appointed on external funds, such as the Oort Fonds, usually for a period of five years (with possible extensions). By the way, the words `buitengewoon' and `bijzonder' are synonyms and mean extraordinary. The Bijzonder/Buitengewoon Hoogleraar are not paid any real salary, only compensations for costs.