Eugene's observations

Again, someone has already written the answer as I would so rather than reinvent the wheel I'll direct you to
http://www.acronis.com/en-us/blog/posts/what-ransomware

My only addition would be to say that in some cases an attack can be thwarted as there are published recovery keys to some forms of attack. This comes about as law enforcement agencies seize the controlling computers and can then publish unlocking keys in some circumstances.

The only sensible protection from this most prevalent form of attack is smart anti-virus protection (e.g. BitDefender) and a considered and verified backup plan.