What is string?
What is parameter?
What is parentheses () ?

Please see the following codes:

<html>
<head>
<script type="text/javascript">
function show_prompt()
{
var name=prompt("Please enter your name","Harry Potter");
if (name!=null && name!="")
{
document.write("Hello " + name + "! How are you today?");
}
}
</script>
</head>
<body>

<input type="button" onClick="show_prompt()" value="Show a prompt box" />

</body>
</html>


name!=null && name!="" is included here.
Here what is the meaning of null?
Here what is the meaning of ?

Thank you.