It can be used however you like, the $ is just another character that is usable in a variable, like _ is.
Generally though, what I've seen it used for:
- library references (jQuery)
- functions that are used in place of native functions (e.g. $timeout in angular)
- internal, "private" methods, usually referenced by double-dollar-signs ($libraryFns.$$private)
- reserved methods of an object that uses a custom getter/setter
- .. I'm sure there are many others interesting uses!