From securityboulevard.com
A type of malicious software aimed to hinder access to a computer system until a sum of money is paid.
In a ransomware attack, victims receive an unsuspicious email attached with a malware. Upon opening the email(which might contain a URL or an attachment like an invoice, but contains the malicious ransomware code), their computers are infected with malicious software.
Organizations and users are mostly not aware that they’ve been infected until they can no longer access their information or until they start seeing messages on their systems advising them of the attack and demanding claims for a ransom payment in exchange for a decryption key. These messages include detailed directions on how to pay the ransom. Payment is usually done with bitcoins as it provides virtual anonymity.