At issue are features including personalized advertising, friend recommendations, messaging and music apps, and customized news feeds.
Yahoo's complaint alleges that Facebook's massive market share is a result of this group of patents, and that Facebook has repeatedly refused to pay licensing fees to use the patented features. Not only is Yahoo seeking unpaid royalties, they're also looking for triple damages (plus interest) and asking the court to stop Facebook from using the patents until they pay up.
Yahoo partnered up with Facebook back in 2010, integrating Facebook features into its sites, but the relationship between the companies seems to have changed under recently-appointed CEO Scott Thompson.
The company has been in decline, losing key board members like chairman Roy Bostock and co-founder Jerry Yang, and it looks like patent trolling is going to be part of Thompson's rebuilding strategy.