The findings are simple:
this is a reference to the html DOM element that is the source of the event.
$(this) is a jQuery wrapper around that element that enables usage of jQuery methods.
jQuery calls the callback using apply() to bind this.
Calling jQuery a second time (which is a mistake) on the result of $(this) returns an new jQuery object based on the same selector as the first one.
Let’s a have a look at how jQuery handles those cases internally.