事件右边的函数的形式问题

1 .1这样写行

oTitle.onmouseup = function() {

document.onmousemove = null;

//这里易错，且不明白，注意

document.onmouseup = null;

1.2 这样就不行，原因啊

oTitle.onmouseup = fnUp;

function fnUp(){

document.onmousemouve = null;

}

2.1  这样写行

document.onmousemove = function(event){

event = event ||window.event;

fnMove(event, disX, disY);

}

2.2 这样就不行

document.onmousemove =fnMove(event, disX, disY);

2.3 但这样就行 

oTitle.onmousedown = fnDown;

function fnDown(event) {

}

原因，请各位大神解答，不胜感激