Well I'm assuming you are fimilar with frames? An iFrame is just an inline frame. That is, it's a method of including another HTML page within a current page. They are most obvious when they appear as a scrollable area of text. Sometimes you won't even know that they are there, for instance the new design of
www.wired.com uses them to include advertisments within the page.
For more detailed information see
www.w3.org/TR/html4/present/frames.html#h-16.5
To include in your page you would use the syntax:
<iframesrc="foo.html">
[Text to appear if frames aren't supported] </iframe>
You can specify height and width attributes for the inline frame and also determine whether it is scrollable.
You can also use the object tag to include a document within another. However there is less browser support for this.
The best way to include another html document is to use a server side scripting language.
No idea about Omniwebs support, I would recommend using Chimera.