CryptoLocker is a piece of malware targeting computers running the Microsoft Windows operating system. It is typically spread as an email attachment, often purporting to be from a legitimate source (including Intuit and Companies House). Some say it is also being spread through the ZeuS botnet.

Once installed on your computer, it systematically encrypts all documents that are stored on your local computer, as well as ones that are stored on mapped network drives and mounted removable storage.

The encryption used is strong, 2048 bit RSA, with the decryption key for your files being stored on a remote server. The odds of you being able to break this encryption is almost nonexistent. If you want to get your files back, CryptoLocker asks for you to fork over some cash; either two bitcoins (At the time of writing, worth almost USD $380) or $300 in either MonkeyPak or Ukash prepaid cards. If you don’t pay within three days, the decryption key is deleted and you lose access to your files forever.
CryptoLocker Is The Nastiest Malware Ever - Here's What You Can Do

Tips:
1. Use common sense when dealing with email attachments
2. Use a good anti-virus/anti-malware software and always keep the signatures updated
3. Backup, backup, backup