some doubts about chown command in linux?

view story

http://superuser.com – I am pretty new in Linux and I have a doubt related to the use of the chown command use. If I have something like this: chown -R root:root /usr/share/MyApplicationDirectory What exactly mean? I think that I am changing the owner of all directory and files content into MyApplicationDirectory assigning them to the root user But why it specify: root:root and not only root? Reading the documenation I have found that the syntax is chown [optuion] [--] user[:group] file1 [file2 …] So I think that the :root is the group. But why? exist a :root group also a root user? (HowTos)