The term "Ping" describes one server sending a notification to another server. The ping typically contains some data about the notification.
A Trackback ping is alternately an attention mechanism or an aggregation tool. When used as an attention mechanism, one server notifies another that there's some attention being paid it. I might send a Trackback ping to a blog entry's URL when I link to or comment on that blog entry.
When used for aggregation, a ping server will be set up to collect and aggregate notifications from a variety of clients. The Newly Digital distributed writing project used Trackback to collect stories from around the web. To join the project, authors sent pings containing the URLs, titles, and summaries of their stories. The Trackback server aggregated all of the stories into a list.
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