I'm by no means the spam expert, I don't really understand it all myself, but referrer spam is basically when a programme (ie: a bot) will forge referrer headers in the browser to get their URI listed in your server logs, in hopes that you display your log on your site and thus it will generate links and what not back to them.
Trackback/ping spam would be similar, a bot would crawl the site looking for trackback or pingback URI's and then try to send trackbacks or pings, again in hope of being listed on your site and generating traffic and links to their site. Having the URI's for tracks and pings listed probably helps them a bit, but they'd figure them out regardless as the format is the same on WordPress (ie example.com/post/trackback).
The above is just my vague idea of what each is, please (anyone) correct me/expand on them. I know there are several plug-ins to deal with all of the various forms of spam, if you're having continual issues with it, maybe check them out.