What is SIDN and where are .nl domains registered?

Who or what is SIDN?
Stichting Internet Domeinregistratie Nederland (SIDN) has been responsible for issuing and registering .nl domain names since 1996. As an individual or company, you cannot apply for a .nl domain with SIDN yourself; such an application must be submitted by a so-called registrar (in SIDN terms, a participant). weboke.nl is an official SIDN registrar.

What exactly is a registrar?
A registrar is a company such as weboke.nl that can register a .nl domain name directly with SIDN. There are also (small) companies that are not registrars but do sell .nl names. They in turn buy them through a registrar, so there is an extra link in the chain. Make sure you register your domain name with a registrar and preferably not with an intermediary.