A worm is a type of malware that replicates itself in order to spread to other computers, often over a network. It can consume system resources which can lead to denial of service. Unlike a virus, it does not need to attach itself to an existing program and often exploits vulnerabilities in network services to spread. Ransomware, while it may spread across networks, is primarily known for encrypting files and demanding a ransom; it is not characterized by its ability to replicate on its own. Spyware is designed to gather information without consent and does not typically replicate itself. A rootkit is designed to provide unauthorized access to a computer system and conceal its presence, not to propagate across networks.