Those tags are evaluated during the Render part of the page’s load cycle. See here for more information.

Displaying (<%= … %>)

Displays the given value as a string.

http://msdn.microsoft.com/en-us/library/6dwsdcf5(v=vs.100).aspx

Displaying with Encoding (<%: … %>)

Displays the given value as string but HTML-Encodes it before.

If you’re sure that the given string is already html-encoded, use it like this:
<%: new HtmlString("<strong>HTML that is not encoded</strong>") %>
http://weblogs.asp.net/scottgu/archive/2010/04/06/new-lt-gt-syntax-for-html-encoding-output-in-asp-net-4-and-asp-net-mvc-2.aspx

Data-Binding (<%# … %>)

Those are resolved when the DataBind method of the control or the page is called.

http://msdn.microsoft.com/en-us/library/ms178366.aspx

Server-Side Comments (<%– … –%>)

Used to comment out code/controls which then won’t be processed.

http://msdn.microsoft.com/en-us/library/3207d0e3.aspx

Server-Side Includes (<!– #include file|virtual=”filename” –>)

Used to insert to content of a given file within the current position.

http://msdn.microsoft.com/en-us/library/4acf8afk.aspx

Embedded Code Blocks (<% … %>)

Used to run some code without returning anything.

http://msdn.microsoft.com/en-us/library/ms178135(v=vs.100).aspx

Expressions (<%$ … %>)

Used for expressions instead of code.

http://msdn.microsoft.com/en-us/library/d5bd1tad.aspx

Directives (<%@ … %>)

Specifies settings for the page or imports and such.

http://msdn.microsoft.com/en-us/library/xz702w3e(v=vs.100).aspx