What do you want to know?
IFrames are an HTML concept - W3Schools is good for learning about how HTML features work -
http://www.w3schools.com/tags/tag_iframe.asp[
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How you use an IFrame is then up to you. Yes, they have uses as they effectively embed one web page as an element in another web page which can give you another way to encapsulate some feature so you can resue it elsewhere - and another way to vary the content within a page.
If you think you have a use for them and are working with ASP.NET, either write the HTML code in your aspx file or add them programatically in your C# / VB code using HtmlGenericControl.