Malware is the umbrella term for all software that is "bad" or malicious. Malicious activity includes:
- Use of the machine as part of a botnet (e.g. to perform automated spamming or to distribute Denial-of-service attacks)
- Data theft (e.g. retrieving passwords or credit card information)
- Installation of software, including third-party malware
- Downloading or uploading of files on the user's computer
- Modification or deletion of files
- Keystroke logging (keylogging)
- Watching the user's screen
- Wasting the computer's storage space
- Crashing the computer
Virus
Infects files, compromises security, takes over control of programs and operating system
Spreads via computers without anti-virus protection software
Spyware
Collects little bits of information at a time about users without their knowledge
Spreads via computers without anti-spyware protection software
Spam
Unwanted emails which may contain malware
Phishing
A fake and fradulent website and/or program designed to steal personal information which appears legitimate and trustworthy
Worm
Spreads itself to other computers on the network (self-replicating)
Spreads via security holes in networks
Trojan
Seems harmless when you first look at it but instead facilitates unauthorized access and control
Rootkit
Enables continued privileged access to a computer, while actively hiding its presence without the owner's permission/knowing
Spreads via exploiting a known operating system/software vulnerability or cracking a password
Backdoor
A method of bypassing normal authentication, securing remote access to a computer, obtaining access to plaintext, and so on, while attempting to remain undetected
Botnet
Collection of the same malware installed on several machines on a large, usually international scale. The risks involved can be anything from sending out spam, keylogging data, and installing fake anti-virus software to presenting the user with CAPTCHAs (to allow whoever created it to automatically make new email addresses etc and continue the chain), or distributing more malware
Ransomware
Malware which investigates your computer for personal files, encrypts them and then demands that you pay a ransom for the decryption key in order to get your files back again
Sources: Wikipedia
