How to Use Loader in SFDC .

<style>
    /* This is for the full screen DIV */
    .popupBackground {
        /* Background color */
        background-color:black;
        opacity: 0.20;
        filter: alpha(opacity = 20);
   
        /* Dimensions */
        width: 100%;
        height: 100%;
        top: 0;
        left: 0;
        z-index: 998;
        position: absolute;
       
        /* Mouse */
        cursor:wait;
    }

    /* This is for the message DIV */
    .PopupPanel {
        /* Background color */
        border: solid 2px blue;
        background-color: white;

        /* Dimensions */
        left: 50%;
        width: 200px;
        margin-left: -100px;
        top: 50%;
        height: 50px;
        margin-top: -25px;
        z-index: 999;
        position: fixed;
       
        /* Mouse */
        cursor:pointer;
    }
</style>  

    <apex:actionStatus id="statusSaveTrip" stopText="">
    <apex:facet name="start">
        <div>
            <div class="popupBackground" />
            <div class="PopupPanel">
                <table border="0" width="100%" height="100%">
                    <tr>
                        <td align="center"><b>Please Wait</b></td>
                    </tr>
                    <tr>
                        <td align="center"><img src="{!$Resource.ImageProp}"/></td>
                    </tr>
                </table>
            </div>
        </div>
    </apex:facet>
</apex:actionStatus>

---------------------------------------------------------------------------------------------------------
<apex:commandButton id="searchBtn" value="Proceed" action="{!search}"  reRender="pMsg,enterprise1" status="statusSaveTrip"/>