Edit in JSFiddle

//fn is the jquery prototype object
//chainability of your plugin can be maintained by returning this, this.each or this.pushStack
(function($) {
    $.fn.changeBackColor= function(options) {
        var settings = {};
        $.extend(settings, this.changeBackColor.defaults, options);
        return this.css('background-color', settings.backColor);
    };
    // defining the defaults for options
    $.fn.changeBackColor.defaults = {
        backColor: 'yellow'
    };
    
    // a plugin that returns a value
    $.fn.average = function() {
        var sum = 0;
        this.each(function() {
            sum += parseFloat($(this).html());
        });
        return sum / $(this).length;
    };
})(jQuery);

$(function() {
    $.fn.changeBackColor.defaults.backColor = 'red';
    $('#div1').changeBackColor();
    $('#div2').changeBackColor({
        backColor: 'green'
    });
    alert("Average is : " + $('div').average());
});

<div id="div1">1</div>
<div id="div2">2</div>
<div id="div3">3</div>
<div id="div4">4</div>