All you need to do to embed something into a post or page is to post the URL to it into your content area. Make sure that the URL is on its own line and not hyperlinked (clickable when viewing the post).
Check out this nice video:
A River Story – the people’s story of the Bristol Avon from source to mouth
I hope you found that video interesting
WordPress will automatically turn that into a YouTube embed when the post is viewed
You can also opt to wrap the URL in the
['embed'] shortcode (don’t type the single quotes, these are to stop WordPress acting on the shortcode). It will accomplish the same effect, but does not require the URL to be on its own line and the “Auto-embeds” setting to be enabled. It also allows you to set a maximum (but not fixed) width and height, like so:
['embed' width="123" height="456"]...[/embed]
If WordPress fails to embed your URL you will get a hyperlink to the URL.
The easy embedding feature is mostly powered by oEmbed, a protocol for site A (such as your blog) to ask site B (such as YouTube) for the HTML needed to embed content (such as a video) from site B.
oEmbed was designed to avoid having to copy and paste HTML from the site hosting the media you wish to embed. It supports videos, images, text, and more.
Can I Use Any URL With This?
Not by default, no. WordPress will only embed URLs matching an internal whitelist. This is for security purposes.
Okay, So What Sites Can I Embed From?
You can use all of these:
• YouTube – only public and “unlisted” videos and playlists – “private” videos will not embed.
Adapted from the WordPress page : <http://codex.wordpress.org/Embeds>