Ransomware is a type of malicious software for data kidnapping. It prevents victims from accessing their data by “encrypting” victims' files on local machine until the ransom (money) is paid. Victims are prompted to pay a certain amount of money (usually in the "untraceable" bitcoins) by a specified deadline in order to retrieve (or decrypt) their data. In most cases, these files cannot be retrieved even the ransom is paid. Once the files are encrypted, there is no way to recover the data, except from your own backups.

Ransomware can attack any machine including Windows PC, Mac and mobile phone, through the following means:

• Malicious email attachments
• Fake free software downloads
• Suspicious websites
• Unsafe links in social media apps like Facebook

Some tips to avoid it can be found from:

http://itsc.ust.hk/services/cyber-security/ransomware