Malware is short for Malicious Software that refers to a group of software that includes viruses, usually Trojan, worms. These are hostile and used to destroy networks, as in terrorist attacks or in committing cybercrime.
Like in a human being, when a virus attacks, it harms every computer that it comes in contact with. It can be forced into the system but what is very common nowadays, is when the user is tricked into downloading the malware into their computer. An example is when a user opens a file from an email and it already contains the virus.
The best form of defense for malware is an antivirus. Once installed in a network or in a PC, it would be able to detect and delete malware like this. Antivirus protects the network against this harmful malware.
