Well, first you're missing the "type" attribute for your buttons - they should be defined like this:
<button type="button" id="btnHide">Hide</button>
<button type="button" id="btnShow">Show</button>
The other problem with this code is that it'll hide all anchors (<a>..</a>) on your page. You should probably give the "jquery" link an ID and only hide this particular link. So, your code imho should look like:
<html>
<head>
<script type="text/javascript" src="JQuery.js"></script>
</head>
<body>
<form id="form1" runat="server">
<button type="button" id="btnHide">Hide</button>
<button type="button" id="btnShow">Show</button>
<a id="jqLink" href="#">JQuery</a>
<script type="text/javascript" language="javascript">
$("#btnHide").click(function(){
$("#jqLink").hide("slow");
});
$("#btnShow").click(function () {
$("#jqLink").show(2000);
});
</script>
</form>
</body>
</html>