In simple terms, ransomware is malicious software that either encrypts your data or otherwise stops you from accessing your own systems. The criminals then demand a ransom in exchange for the decryption key, although, of course, there is no guarantee that the key will work and you will get your data back. Many victims have paid up but not gotten their data back.
Compared to the straightforward WannaCry-style “compromise and encrypt” attacks of a few years ago, attackers are now taking a more sophisticated multi-vector approach. Attacks still often start with a spear-phishing email, but today’s ransomware attacks aren’t triggered immediately when the target clicks the malicious link.