A malicious program that usually requires some action on the part of a user in order to infect a computer; for example, opening an infected attachment or clicking on a link in a rigged email may trigger a virus to infect your computer.

Self-replicating malware that, for instance, hunts down unprotected computers and recruits them for criminal or other malicious purposes. Unlike a virus, worms do not require any action on your part in order to infect your computer.

A Trojan horse, in software terms, refers to a malicious piece of software that is hidden inside an apparently useful and innocent application. Users systems generally become infected by trojan horses when they install software or a fake "patch" from an Internet download or an email attachment.

Remote Access Trojan Horses (RATs)

Malware that gives an attacker ongoing remote control of your computer, allowing them to browse files, install additional malware, or use your network connection for other attacks.

Spyware and adware are small software programs that get stored on your computer without your permission, usually when you visit a website, sign up for "free" mailings, or download "free" software. These programs gather information about your computer use, web browsing, online purchasing and send it to sites that may sell it to others.

Rootkits are designed to give an attacker "root" or administrative access to a victim's computer. With root access, an attacker can perform a variety of tasks on the computer including browsing the infected computer's hard drive, making new user accounts or changing existing ones, or even using the computer's network connection for nefarious purposes.

Fileless Malware / “Living off the Land”

Malware that abuses legitimate tools already on your system (for example, PowerShell or WMI) so little or no malicious file is saved to disk, which makes it harder to detect and remove.

Cryptojacking (cryptominer malware)

Software that hijacks your computer’s processing power to mine cryptocurrency, which can make your system run hot and slow, even when you aren’t actively using it.

Malicious Browser Extensions

Add-ons that look helpful but can read your web activity, inject unwanted content, or steal login data; only install extensions you truly need and review their permissions regularly.

Botnets

Groups of infected computers controlled by attackers and used for spam, credential stuffing, and denial-of-service attacks; a single infected device can silently become part of a much larger attack.