from: http://www.tamba2.org.uk/wordpress/ping/

Sending Pings and Trackbacks

This is not telling you what a Ping is, what a Trackback is or when and why you should use them.
If you want that information, head over to the WordPress Codex.
This page will tell you how to do them.
The example url I use is this one: http://wordpress.org/development/2006/04/wordpattern

Pinging
Pinging a post on another blog from your post is very easy. All you do is link to it.
The link in this screenshot goes to the post above.

That will cause a ping to be sent. Easy.

Self-pinging
If you link to a post you made earlier on your blog, you will ping yourself. Some people like this, others do not. A reason for you liking it is that if someone reads your old post and the comments, they will see that you have pinged the post and may follow that to see what else you have to say on that subject. You can choose whether to or not on each occasion.

Using the above url, if Matt wanted to link to that post in a later post he would use this as a link:
<a href=”http://wordpress.org/development/2006/04/wordpattern/”>
That should send a ping.

If Matt did not want to ping his own blog though, he could link to it like this:
<a href=”/2006/04/wordpattern/”>
This relative url will still link if you click it, but will not send a ping.

Sending a Trackback
A bit more work is needed for this.
Still using the same post on the WordPress development blog, you would start the same way:

Now you would go to the post on the blog and look for the ‘Trackback URI’ or somewhere that Trackback is written.

Once you have that link you need to find it’s location. In Firefox this would be Right-click on the link and ‘Copy Link Location’

Back on your blog, scroll down from the Write area to the ‘Trackbacks’ box and paste the whole link location into that box. It MUST start with http:// and it MUST end with /trackback/.

Now post and the trackback will be sent.