If you'd like to register a domain to make certain that nobody else is going to take it, but you haven't built the website for it yet, you may park it. This is a service that registrar companies provide if a domain address isn't connected to any web or email hosting service. In this way, you'll be able to protect a brand name, for example, and you'll own the domain address in question although it will not open any content. If you'd like, you can pick some default template that the registrar provides, like For Sale or Under Construction, or you can forward the domain name to an alternative web address. Your second option is very helpful if you own a few domain names, but you want each and every one of them to open the same site. For example, you could register domain.net and domain.org, then park them and direct them to domain.com. In this example, you'll need hosting for the third domain name only and the traffic to the other ones is going to be redirected to it.
