The encyclopedia proper is themain namespace and contains all of Wikipedia's encyclopedia articles. All of the other namespaces have prefixes, which must be included (followed by a colon) at the beginning of links to pages in those namespaces in order for those links to work. Otherwise, the links will point to the main namespace, which has no prefix.
Currently, Wikipedia has 30 namespaces: 14 subject namespaces, 14 corresponding talk namespaces, and 2 virtual namespaces.
The namespaces (by prefix) are:
The virtual namespaces are:
In any given namespace there can only be one page with a particular name.