Actually, it's far from informal. It's a very basic aspect of how the JQuery code is organized.
For purposes of this discussion, I will ignore the fact that Javascript actually lacks an explicit namespace feature. But there are means nevertheless of effectively creating namespaces in Javascript. That is beyond the scope of this question, though.
All of JQuery's functions, by default, are in the
jquery namespace, and start with
jquery. $ is a convenient synonym for
jquery. This can be overridden, in case this creates a conflict with something else, and, in fact, there are other packages (the most common being Prototype) that similarly use
$ as a synonym.
JQuery Mobile's functions, in turn, are, by default, in the
jquery.mobile namespace. They can also be referred-to by the more convenient synonym,
$.mobile.
See more details here:
http://docs.jquery.com/Using_jQuery_with_Other_Libraries
