Simply put, this type of virus is devastating. CryptoWall (and the rest of the CryptoLocker variants) will encrypt (lock) your files and demand a ransom to get the key.
Files affected are usually photos, documents, music, and movies.
So far, there’s no way to crack the encryption (unlock the files) without paying the ransom, and there’s no guarantee the hackers will give you the key even when you’ve paid. The ransom can be anywhere from $400-$600 and with every new variation it’s going up.
The best way to prevent infection is to have a current backup of your files. Once the virus is removed (or once you wipe and reload the computer), you can restore your files. Most antivirus programs can remove the virus, but that won’t help you get your files back.