Nevermind, found out already ;)
If someone else is asking himself the same thing and has troubles to look it up:
<% %> is the normal Syntax for server-side script (processed before the page is rendered to the client).
So something like <% Response.Output.Write("<b>Hello</b>"); %> will just write the bold 'Hello' Text to the response instead of the original statement.
<%= %> is an abbreviation for the upper statement, i.e. <%="<b>Hello</b>" %> would just write the same text to the response...
<%=myServersideObject.ClientID%> would write the ClientID of the object to the repsonse (given the fact, that the property 'ClientID' is a string).
At last <%# %> is DataBinding-Syntax (therefore one has to do DataBind()) somewhere on the page when using r.a.d.window...
Maybe this helps someone else who is new to ASP.NET too! Everyone else will know this anyway i guess ;-)