I've been blogging for a few years now and I never quite understood the difference between trackbacks and pingbacks until just now. The trackback, as explained above, is more of a conscious attempt on the part of the user to put a link between two sites via a trackback submitter.
A pingback is more of an automatic connection that's created when someone places a link to a given post in their own post or site. Depending on what platforms each site is using, the existence of the link itself may create a connection to the linked post, which can then be moderated like a comment or trackback.
Some platforms have automated trackbacks so much, though, that the difference between pingbacks and trackbacks is blurring.