Why are some names reserved?
Some page names and/or URL segments are reserved due to technical reasons. For example, the keyword "article" is bound to indicate that the URL segment that follows refers to a news article. Likewise a page with Pagetitle "News" is bound to behave as a container for the tt_news application and automatically links to the RSS feed (which won't exist if you don't use the tt_news application).
This behavior cannot be changed on an individual site basis.
What names are reserved?
The following list of page titles/URLs are not recommended for use unless it is for the purpose they are bound to (for example, Search must be used as the search page).
The best advice is to avoid these reserved keywords for both page names and page URLs.