What is malware ? - Security Term

Malware is a category of malicious software programs that includes viruses, worms and Trojans. Malware spreads via email, the Web, and devices like USB drives. Different malware file components are often used together to create blended – and effective – Web threats.
Viruses have the unique ability to replicate, and like biological viruses, can spread quickly. Viruses typically damage your PC by reformatting your hard drive or corrupting files.

Worms spread copies of themselves via email and instant message attachments, and harm networks by consuming bandwidth.

Trojans perform malicious actions but have no replication abilities. Like the original Trojan horse, these programs may arrive as a seemingly harmless file or application but actually have malicious intent within their code.

source: us.trendmicro