Trojan.Cryptowall is a Trojan horse that encrypts files on the compromised computer. It then asks the user to pay to have the files decrypted.
The threat typically arrives on the affected computer through spam emails, exploit kits hosted through malicious ads or compromised sites, or other malware.
Once the Trojan is executed on the compromised computer, it creates a number of registry entries to store the path of the encrypted files and run every time the computer restarts. It encrypts files with particular extensions on the computer and creates additional files with instructions on how to obtain the decryption key.
This threat family attempts to convince the user to pay money in order to get the key to unlock their files. It uses a variety of different techniques in order to encourage the user to pay the ransom.
More information on Trojan.Cryptowall is available in the threat family writeup.