Malware

Malware, short for malicious software, is an umbrella term used to refer to a variety of forms of hostile or intrusive software, including computer viruses, worms, trojan horses, ransomware, spyware, adware, scareware, and other malicious programs. It can take the form of executable code, scripts, active content, and other software. Malware is defined by its malicious intent, acting against the requirements of the computer user — and so does not include software that causes unintentional harm due to some deficiency.

Programs supplied officially by companies can be considered malware if they secretly act against the interests of the computer user. An example is some code created by a very large electronic company that was basically a Trojan embedded into CDs. When executed, this code would silently install and conceal itself on purchasers' computers with the intention of preventing illicit copying. However, it also reported on users' listening habits and unintentionally created vulnerabilities that were exploited by unrelated malware.

Software such as anti-virus and firewalls are used to protect against activity identified as malicious, and to recover from attacks.

https://en.wikipedia.org/wiki/Malware

Malware by Category

Trojan Horses

As you can see from the pie chart above, Trojan Horses are the most common type of malware attack. For this reason, I am going to linger on it specifically for a bit longer.

A Trojan is a malicious computer program that misrepresents itself to appear useful, routine, or interesting in order to persuade a victim to install it. The term is derived from the Ancient Greek story of the Trojan Horse used to invade the city of Troy by stealth.

Trojans are generally spread by some seemingly innocuous action taken by a user, for example where a user is duped into opening an email attachment disguised to be unsuspicious, (e.g., a routine form to be filled in), or by an embedded link in the message. Although their payload can be anything, many modern forms act as a backdoor, contacting a controller which can then have unauthorized access to the affected computer. While Trojans and backdoors are not easily detectable by themselves, computers may appear to run slower due to heavy processor or network usage.

Unlike computer viruses and worms, Trojans generally do not attempt to inject themselves into other files or otherwise propagate themselves.

Unauthorized Remote Access

As you read above, a common attack methodology is to dupe a user into downloading a script that opens up a backdoor and allows the hacker full access to your computer. The full range of nefarious opportunities "full access" entails cannot be overstated. 

If a hacker gains full access to your computer, they are not only able to use it — quite easily, I might add — to acquire your personal information or sensitive business data, but they are also able to use your computer as a jumping off point for illegal activities, essentially framing your IP address (or in other words, you) for their crimes.