Question about onEnterFrame

Maybe this is a stupid newbie question but if someone could explain me

what is the difference on using in actionscript

mc1.onRollOver = function() {
	onEnterFrame = function () {
		mc1._alpha < 100 ? mc1._alpha += 5 : delete onEnterFrame;
	};
};

mc1.onEnterFrame = function () {
	this.onRollOver = function() {
		mc1._alpha < 100 ? mc1._alpha += 5 : delete onEnterFrame;
	}
}

If someone could explaine me in actionscript terms. Thanks in advanced :red: