<% ... %> The most basic inline tag, basically runs normal code: 
 
<% if (User.IsInRole("admin")) { %> You can see this <% } else { %> You are no admin fool! <%} %> 
http://msdn2.microsoft.com/en-us/library/ms178135(vs.80).aspx 
 
<%= ... %> Used for small chunks of information, usually from objects and single pieces of information like a single string or int variable: 
 
The Date is now <%= DateTime.Now.ToShortDateString() %> The value of string1 is <%= string1 %> http://msdn2.microsoft.com/en-us/library/6dwsdcf5(VS.71).aspx 
 
note: <%= is the equivalent of Response.Write() - Courtesy of Adam from the US,thanks! 
 
<%# .. %> Used for Binding Expressions; such as Eval and Bind, most often found in data controls like GridView, Repeater, etc.: 
 
<asp:Repeater ID="rptMeetings" DataSourceID="meetings" runat="server"> <ItemTemplate> <%# Eval("MeetingName") %> </ItemTemplate> </asp:Repeater> 
 
http://msdn2.microsoft.com/en-us/library/ms178366.aspx 
 
<%$ ... %> Used for expressions, not code; often seen with DataSources: 
 
<asp:SqlDataSource ID="party" runat="server" ConnectionString="<%$ ConnectionStrings:letsParty %>" SelectCommand="SELECT * FROM table" /> http://msdn2.microsoft.com/en-us/library/d5bd1tad.aspx 
 
<%@ ... %> This is for directive syntax; basically the stuff you see at the top your your aspx pages like control registration and page declaration: 
 
<%@ Page Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" Title="Untitled Page" %> <%@ Register TagPrefix="wp" Namespace="CustomWebParts" %>
http://msdn2.microsoft.com/en-us/library/xz702w3e(VS.80).aspx 
 
 
 
 
 
<% ... %> This is a server side comment, stuff you don't want anyone without code access to see: 
 
<asp:Label ID="lblAwesome" runat="server" /> <% sometimes end users make me angry %> <asp:LinkButton ID="lbEdit" Text="Edit" OnClick="Edit_Click" runat="server" /> http://msdn2.microsoft.com/en-us/library/4acf8afk.aspx