AJAX stands for Asynchronous JavaScript and XML.

It is a cross browser based thing that reduces the response time or you can say postback.

These controls add script on the page that will be executed by the browser.

The ScriptManager Control

<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>

This is the script tag required to execute AJAX controls in the .NET framework.

The UpdatePanel Control

<form id="form1" runat="server">
   <div>
      <asp:ScriptManager ID="ScriptManager1" runat="server" />
   </div>
   
   <asp:UpdatePanel ID="UpdatePanel1" runat="server">
      <ContentTemplate>
         <asp:Button ID="btnpartial" runat="server" onclick="btnpartial_Click" Text="Partial PostBack"/>
         <br />
         <br />
         <asp:Label ID="lblpartial" runat="server"></asp:Label>
      </ContentTemplate>
   </asp:UpdatePanel>
   
   <p> </p>
   <p>Outside the Update Panel</p>
   <p>
      <asp:Button ID="btntotal" runat="server" onclick="btntotal_Click" Text="Total PostBack" />
   </p>
   
   <asp:Label ID="lbltotal" runat="server"></asp:Label>
</form>

Update panel when used with a container and the controls which are placed inside update panel then it will reduce their postback time.