Terminologies

Label
arbitrary string consisting of alphanumerics and dash ("-")
upper/lower case does not matter (www = WWW)
underscores ("_") are INVALID
associated with each 'node' of the tree structure
Domain
a sub-tree starting from a node
domain name: string from an apex node of the domain to the root, with "." as delimiter
toshiba.co.jp.
kame.net.
so-called "host name" is a speical case of domain name
www.toshiba.co.jp.
kame202.kame.net.
foo.bar-baz.com.