jQuery.fn - How does it work?

I'm having a lot of fun extending jQuery, but some thing looks much obscure.
One of them is why i need to use
jQuery.fn.extend({
'myPlugin': function(opts) { ... }
})
And why it will not work if I just make something like
jQuery.myPlugin = function(opts) { ... }
Any help?
--
Reinaldo de Souza "JuniorZ"
http://reinaldojunior.net
-- Non ducor, duco