Iframes is also defined as an "Inline Frame". Inline Frames are those frames that are embedded in any part of the main page. It made its debut way back when Microsoft had released 'Internet Explorer v.3.0'. In the initial stages the usefulness of Iframes where limited to local applications which were compatible to the Microsoft browser.
Over a period of time realizing the significance of Iframes most of the other well-known browsers (Netscape, Opera) have also added the plug-in to make the Iframes compatible to their browsers. So now except for people who are still using old browsers Iframes otherwise seems to have put its mark in the Internet World.