CryptoLocker can access your computer by a botnet set up by many viruses or the newly updated GameOver ZeuS. Not much is known about this malware yet. It was first discovered by Dell SecureWorks on September 2013.
CryptoLocker's main goal is to display a ransom of an unknown amount of money based on how much you have. It basically encrypts your files and to get the key to it, you need to pay a ransom. It stores 2 keys, the private key which unlocks your files, and the public key that encrypts your files. It says that it has a 2048-Bit key but it actually only has a 1024-Bit key, which is still hard to break.