One definition is people that love to get into arguments on the Internet
A second definition is an individual or group engaged in collecting money from unsuspecting victims for "crimes" they did not even know they may have committed. (see Patent Trolls, Image Trolls)
If hiring a still photographer, be sure to get a "work for hire" agreement that provides you with ownership of the images.
If you use public domain sites, be sure to examine the rights closely and document your source. (And even then you are not 100% guaranteed to be cleared.)
Masterfile
If you host sites for others, or if your site has a way for others to post content, it is worth it to register to become a DMCA agent.
Costs $105 – But you have "safe harbor" if clients or visitors infringe. The copyright holder must issue a DMCA takedown notice.
http://www.wired.com/threatlevel/2010/10/dmca-righthaven-loophole/
Visit: http://extortionletterinfo.com