A parked domain name is one that is taken, however it's not linked to some other service - web or email hosting. Put simply, such a domain name won't open any actual content if you type it within a browser, but nobody else can take it as you've already registered it and you are its owner. The advantage of getting a domain name and parking it afterwards is the fact that you’re able to keep it and not stress that someone else may register it meanwhile if, for example, the site for it is not ready yet. You could set a temporary Under Construction webpage for a parked domain while you're working on your web site, or forward it to some other web address - temporarily or permanently. The latter option can be used if you have already registered domain.com, for instance, and you get domain.net and domain.org, so as to protect a brand name or a trademark. In this case, the .net and .org domains can be parked and forwarded to the .com as you do not need individual websites for them.