Apex refresh part of the page(use reRender attr...

<apex:actionStatus id="loading" layout="block">
<apex:facet name="start">
<div class="overlayBackground displayBG">
<img src="/img/loading.gif" id="loading" class="loadingImg"/>
</div>
</apex:facet>
</apex:actionStatus>

style:

<style type="text/css">
.displayBG
{
border: none;
width: 100%;
height: 100%;
display: block;
position: fixed;
text-align: center;
}
.loadingImg
{
height: 20px;
width: 20px;
margin-top: 200px;
}
</style>