$ is the jQuery object itself, which when called implements a whole pile of different interfaces. $('string') runs a selector or constructs a node; $(domElement) wraps an element… and $(a_function) is a convenient short hand for $(document).ready(a_function). See the jQuery API docs for (much) more information.
A note in passing: $(function () { ... }) is shorter, but if you ever want to search for all of your on-ready events, you might be wishing that you had .ready to search for 🙂
There is no difference.
One is a convenient shorthand that calls the other internally.
From the jQuery docs:
A shorthand for
$(document).ready().
Allows you to bind a function to be
executed when the DOM document has
finished loading. This function
behaves just like$(document).ready(),
in that it should be used to wrap
other$()operations on your page that
depend on the DOM being ready to be
operated on. While this function is,
technically, chainable – there really
isn’t much use for chaining against
it.You can have as many
$(document).ready
events on your page as you like. See
ready(Function) for details about the
ready event.