The forum has the ability to embed all sorts of videos from all sorts of places around the internet directly in your post.
The full instructions can be found here.
The most commonly embedded video is YouTube so I will go through the example of how that one works.
Here is a URL or address of a YouTube video:
http://youtube.com/watch?v=pmOmT7a8NB4
The video ID (the part we need) is bolded:
http://youtube.com/watch?v=pmOmT7a8NB4
Sometimes there will be additional stuff after it like this:
http://www.youtube.com/watch?v=pmOmT7a8NB4&eurl=
But we still only need this part in bold:
http://www.youtube.com/watch?v=pmOmT7a8NB4&eurl=
After we take that part out we put "youtube" (no quotes) in curly brackets {} before it and "/youtube "(no quote) in curly brackets {} after it like so:
and the result is: