Ransomware is a type of malware that encrypts the victim's files and requires payment, often in cryptocurrency, to provide the decryption key necessary to regain access to the affected data. This process aims to extort money from victims by denying access to their own files until the ransom is paid. The other options do not match this behavior: a Trojan horse is a type of malware that misleads users of its true intent; spyware is designed to gather information covertly; adware displays unwanted advertisements.