Ransomware is a form of malicious software (or malware) that, once it’s taken over your computer, threatens you with harm, usually by denying you access to your data. The attacker demands a ransom from the victim, promising — not always truthfully — to restore access to the data upon payment. Users are shown instructions for how to pay a fee to get the decryption key. The costs can range from a few hundred dollars to thousands, payable to cyber criminals.

One of the most common delivery systems is phishing spam — attachments that come to you in an email, masquerading as a file you should trust. Once they’re downloaded and opened, they can take over the victim’s computer, especially if they have built-in tools that trick users into allowing administrative access. Some other, more aggressive forms of ransomware, like NotPetya, exploit security holes to infect computers without needing to trick users. The good news is companies like McAfee, Microsoft and SonicWall are all aware of the many types of phishing schemes that are rampant world wide.