A patent troll is an informal legal term used to describe an entity which owns a patent, does not manufacture or produce anything that uses the patent, but agressively pursues others at law who they claim are using the patent. At law, there is nothing improper in this arrangement, but many modern manufacturers complain that the application of patent law forces them to settle cases when patent infringement is claimed rather than litigating the matter.
One better known example was a company that purchased some of the patents underlying the UNIX operating system. They then pursued commercial users (such as companies running web servers) of the Linux operating system (which was largely based on UNIX) stating that they were violating their patents. Supporters in the Linux community protested as Linux is an open source piece of freeware and argued the patents were not, in fact, beign infringed or were invalid from the beginning. However, the cost of litigating the dispute would cost more than paying the claimed licensing fee, and a failure by a Linux user to defend their use could result in punitive damages.
By contrast, Apple recently pursued its rival Samsung over alleged patent infringement, but could not be called a patent troll - Apple uses those patents in its own products and wants to ensure that competitors either negotiate licensing agreements or cease using their patents.