HTML5
<!DOCTYPE html>
<html lang="ja-JP">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width,user-scalable=yes,initial-scale=1.0,minimum-scale=1.0,maximum-scale=1.5">
<title></title>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script>
window.addEventListener("load",function(){
document.querySelector("input[type='button']").addEventListener("click",function(){
document.querySelector("input[type='text']").value = "";
});
});
</script>
<body>
お名前：<input type="text"><input type="button" value="クリア">
</body>
</html>
jQuery
<!DOCTYPE html>
<html lang="ja-JP">
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width,user-scalable=yes,initial-scale=1.0,minimum-scale=1.0,maximum-scale=1.5">
<title></title>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js"></script>
<script>
$(function(){
$("input").val("入力してください")
.css("color","#CCCCCC")
.focus(function(){
$(this).val("").css("color","#000000");
});
});
</script>
<body>
お名前：<input type="text" vlaue="">
</body>
</html>