The exclusive rights held by a copyright owner are the right to make copies and allow others to make copies of the work, the exclusive right to make derivatives of the work, the exclusive right to market the work, the exclusive right to display the work and the exclusive right to perform the work.
If anyone infringes on any one of these exclusive rights, the copyright owner is entitled to sue for infringement.
In addition, if the copyright holder transfers, in writing, one or more of his exclusive rights in his copyrighted work to another, then the holder of these transferred rights is also entitled to sue for copyright infringement.
In the Righthaven cases, the copyright holders only assigned "a right to sue" to Righthaven but did not transfer any rights in the copyrighted works to Righthaven. In fact, in the assignment the copyright holders explicitly retained all rights to their copyrighted works. Righthaven was not entitled to sue for copyright infringement because Righthaven never held any rights in the copyrighted works.
The Righthaven cases should not, however, be confused with all other copyright infringement cases, even those that are currently being likewise-labeled as "shams" and "scams."
That said, it would still be wise for anyone who receives any copyright infringement notice to make sure that the person who, or entity that, is claiming infringement actually holds rights to the copyrighted work and has standing to sue.
