Local or Remote WebForms Application
You can use AJAX if you are trying to communicate with an external (or local) application.
Local WebForms Application
If by "application" you are actually referring to the back-end (code-behinds, etc.) of your WebForms project, then the other thing to look into are "server tags," otherwise known as "bee-stings." Here are just a few exsamples:
<% %>
<%-- --%>
<%# %>
<%= %>
Additionally, you can use event handlers for things like server-side button or anchor clicks, dropdownlist value changes, etc. You can make standard HTML controls server-side by adding the runat="server"
attribute, or you can use .NET's WebControls (though they will still have to have the runat="server"
attribute). Examples of these would be:
Front End
<button runat="server" onserverclick="btn_click">Click me</button>
...
or
...
<asp:Button runat="server" OnClick="btn_click">Click me</asp:Button>
Back End
protected void btn_click(object sender, EventArgs e)
{
...
}