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

When using a AutoCompleteExtender from the AjaxToolkit, Firefox may show it’s own AutoComplete List over the one you want. To disable that, you need to disable autocomplete on the Field or the entire Form.

To do that, just add an autocomplete="off" to the Textbox in the Markup Code or add the Attribute on Runtime like this:

Textbox1.Attributes.Add("autocomplete", "off");