Malicious software or malware comes in different forms such as adware, Trojan horse, viruses, spyware, and ransomware. Adware collects user data to sell to third parties and advertisers, Trojan horse tricks users into installing other forms of malicious software, viruses infect computer systems and multiply throughout the operating system, spyware tracks user behavior and keystrokes to capture sensitive data, and ransomware encrypts user data to hold for ransom until payment is made. Malware can also come in undetectable forms like fileless malware and complex variants like polymorphic malware, which are harder to prevent, detect, or remove.