Ransomware is a type of malware that get into a computer or network and then encrypts/locks files and data, then demands ransom payment to release them.
Ransomware attacks often present themselves in the form of phishing. The victim will click on an attachment that appears legitimate, but is a malware file. The email 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.
For several years, these attacks appeared in the form of spam type emails, but they've become more targeted, using "spearphishing" techniques. A spearphishing email can appear as if it's 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 email 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.