The Wall is a space on every user's profile page that allows their friends and connections to post messages for the user to see.  To post on someone's wall, scroll down on their profile page to the Wall box.  Type in the text box and click the "Add a Wall Post" button to submit your post.

You can also post Youtube videos on people's Walls by typing {youtube}...{/youtube}

Let's say the Youtube link is http://www.youtube.com/watch?v=voYkKxSPkfw&feature=related

To paste the Youtube video on the Wall, you do this

{youtube}voYkKxSPkfw{/youtube}