As Defined in Wikipedia
Ransomware, or extortive malware that holds users' data to ransom.
By the way so far .. have not had any of these issues with OS X.. Not to say Apple is better But... OS X is a variant of *nix operating systems. So loop holes are not as accesable and wide spread as MicroSoft Operating systems.
Basically is software or code that is designed to require you to purchase their software to get your computer back.. This could disable items via your registry including but not limited to Anti-Virus, Anti-Malware, Anti-Spyware, Task Mananger, Regedit, and even msconfig. In effect entirely disabling your system from usage until you pay for their product. (even though it was their application that caused the problem in the first place) Nice huh? Actually it's a brilliant way to make $$ till they get caught.. (at least I hope they get caught).
As posted earlier today I had a run in with How to Fix Windows Police Pro
This is a nasty little program that reminds me of the commercials from Apple as follows.. Which by the way make me laugh my A$$ off.
And here is the rest of it.