Ransomware is malicious software that gets on your computer and encrypts your files so that you can no longer access them. The program will tell you that your files are encrypted and the only way to get them back is to pay for a decryption key. It may also say that you have a deadline before the key is destroyed, rendering all of your encrypted files useless.
The best way to deal with this kind of threat is to have antivirus software installed on your computer; just be sure your antivirus software is up to date. Also verify that you have the latest updates for your computer. Have a backup in place so that if your computer is hit with such an attack, you can recover your files.
Recovering From Ransomware
If this kind of software has made its way on to your computer and encrypted your files, search for recovered decryption keys and tools for the specific ransomware on your computer; Kaspersky has built a decryption tool to recover files encrypted by the CoinVault and Bitcryptor ransomware. Of course the last resort is to pay the ransom and get the key. In fact, this is exactly what the FBI recommends.
Also, it was just reported that ransomware has been targeting webhosts, so if you have a website, be sure you have regular backups.