A domain name is a unique address and has an important function on the Internet.
By use of domain names, it is possible to effectively search for the website of the company and / or person you're looking for. A well chosen domain name is easy to remember and can ensure that you will be found quickly by the search engines on the Internet. When you have a trademark, it is wise to use this name as a domain name. If you do not, the competitor can register your business name and so take advantage of your reputation.
Originally a domainname has a link to the unique number every computer has on the internet, the IP-adress (Internet Protocol). THe IP-adress consists of a digit combination, but because words have more meaning than numbers, it was chosen to translate these numbers to names.
For companies we advise to register your trademark in multiple ways, for example yourtrademark.nl and yourtrademark.com. In general, only one or two domain names used for your website. Any other domain names will be regeistered for you, but not used. This allows you to protect yourself as a company against something called "domain hijacking". You can also have these domains send you to an other domain.