Ransomware is a type of malware that get into a computer or network and then encrypts/locks files and data, and then demands payment, "ransom", to release them.

Ransomware attacks often present themselves in the form of phishing.  The victim will click on an attachment that appears legitimate, but it the actual malware file. Or the e-mail might contain a legitimate-looking URL, but when a victim clicks on it, they are directed to a website that infects their computer with malicious software.

And whereas these attacks for several years appeared in the form of spam type e-mails, they've become more targeted, using "spearphishing" techniques.  Since e-mail filters have improved so much and block most spam mails, as spearphishing e-mail can come in the form of an e-mail from a person you know or company you do business with.  

Never click on a link or file you do not know/recognize, particularly if it is unexpeted or unsolicited.  If any e-mail seems unusual or suspicious, delete it immediately.  Back-up your data regularly, and maintain backup data storage seperate from your primary computer and/or network.