How hide div in javascript?

Question

<script type="text/javascript">
function HideDiv(id) {
    document.getElementById(id).Style.display = "block";
}
</script>

Answer 1

document.getElementById(id).style.display = "none";

Answer 2

The correct syntax is element.style, not element.Style. "style" is a CSSStyleDeclaration object, "Style" is undefined. As such, for you, the code you need is:
document.getElementById(id).style.display = "none";
Helpful reference: http://www.w3schools.com/css/pr_class_display.asp