iFrames are often used to embed external content on pages e.g. youtube videos, audio.
Embedded Content
The default way of embedding external content into a page is using iFrames, there is also an older approach using frames. These pages allow you to explore the different approaches.
iFrames With External Site Content
iFrames are used to embed external content on pages e.g. youtube videos, audio. But site permissions are also important.
Frameset is deprecated and harder to work with.
iFrames are nested pages within the body of the page.