Ransomware is a type of malware (virus) that blocks access to your files, and then demands that you pay a ransom in order to get them back. This is usually done by encrypting the files, requiring a password to decrypt the data. The password is available to you for a small fee, usually ranging from a few hundred dollars to a few thousand.
Cybercrime is morbidly fascinating to me. Don't get me wrong, these guys are thugs and I have dedicated my professional life to squashing them like the cockroaches that they are. But I am fascinated by the fact that they run "real" businesses. They spend a ton of money on R&D. They fight fierce competition for market share.