Crypto-ransomware encrypts the files on your computer so that you can't access them without a decryption key. In exchange for the decryption key, a ransom fee is demanded, which is usually around $300 to $500 per computer. Payment is often demanded in Bitcoins, a virtual currency that is difficult to trace.
You may encounter ransomware in various ways: by opening a malicious email attachment, visiting a compromised or malicous website, or clicking on a compromised ad. You may not even realize you're being infected – until your files are locked up, of course.
Nobody wants to have to pay the ransom. Paying may be one way to regain control of your computer and data, but there are also cases of victims paying and still not regaining control of their files. The best remediation begins before you ever get hit, by taking regular backups. That way, if you do get attacked, you can relax and restore from backups.
If your files have been hijacked and you don't have backups, it's worth checking if there is a decryption tool for the ransomware you've been hit with. You can also share your situation on a help forum like Bleeping Computer, where there are threads to help with many families of ransomware. We also recommend that you report the crime to the relevant authorities, typically the police.
F-Secure SAFE protects against all known ransomware threats, and it can block brand new zero-day threats as well.
Store them offline so they can't get infected, and test them from time to time to make sure they really work.
For help with this, consider using a tool that identifies old software versions and suggests updates.
Be aware that criminals spoof the emails of big brands such as Amazon or FedEx. Type links into your browser rather than clicking from the email. Be extra careful with attachments requesting you to enable or allow something — macros, editing, content, etc.
Disable commonly exploited ones such as Flash Player and Silverlight when you're not using them. You can do this through your web browser under the plugin settings.