Here's a site that is completely focused on blog 'pinging'. http://pingomatic.com/about/ so if you take a read there, you should get a better idea. The bare details, it lets another blog know either 1) you posted a link to the other blog or 2) that you posted a new entry on your blog.
It's a matter of letting people know you've updated your blog and/or written something about their post (in the case of a trackback to another blog)