"el" is HTMLElement
"$el" is jQuery
lets say that you do this
var myel = this.el; // here what you have is the html element,
//you will be able to access(read/modify) the html
//properties of this element,
with this
var my$el = this.$el; // you will have the element but
//with all of the functions that jQuery provides like,
//hide,show etc, its the equivalent of $('#myel').show();
//$('#myel').hide(); so this.$el keeps a reference to your
//element so you don't need to traverse the DOM to find the
// element every time you use it. with the performance benefits
//that this implies.
one is the html element and the other is the jQuery object of the element.