The litteral meaning of malware is "malicious software", it is used to qualify a long list of threats including trojans, worms and viruses.
Malware is usually stealthy to avoid detection, but some malware such as ransomware will disrupt the functioning of the PC.
Stealthy malware is more common because today's malware is primary aimed at stealing private information such as login information. Malware is very profitable for cyber criminals, targeted attack are constantly growing for that reason.
Trojan Backdoors: Backdoors are meant to give a remote hacker control of the infected PC. The hacker can do pretty much whatever he wants using the computer, the same way the legitimate user can. Backdoors are one of the most common types of Trojans and are designed for stealth to avoid raising suspicion.
Trojan Exploits: Exploits, as their name indicate, are designed to exploit. They generally use vulnerabilities found in software’s used on the computer. Most software contain vulnerabilities which is why software companies often update their software, however most users do not download those updates which leaves them vulnerable to exploits.
Trojan Rootkits: Rootkits are similar to a smoke screen, they are designed to hide malicious activities on infected computers so that the malware infecting the machine avoids detection. Rootkits allow hackers to benefit from their malware for a longer period of time.
Trojan Bankers: The primary purpose of Trojan bankers is to steal banking systems information’s such as logins, credit or debit card numbers. These threats will gain in importance as more and more people use the internet to transact with various companies.
Trojan DDoS: DDos stands for “Denial of Service”, such attacks are due to the action of this type of Trojan. What a Trojan DDos does is infect thousands of computers and use them all at the same time to send requests to a website in order to disrupt the site so that it goes into a denial of service.
Trojan Downloader:Downloaders are coded to install malware on the infected computer from a remote server. This kind of threat may install all sorts of programs such as adware or more serious threats like keyloggers.
Trojan-FakeAV: FakeAv are designed to replicate the action of a legitimate antivirus in order to extort money from users. A legitimate antivirus shows real malware results whereas a FakeAv will show fake results in order to scare the user into purchasing a license to remove those fake threats.
Trojan GameThief: This type of threat is specifically aimed at stealing gaming information from players of online games.
Trojan-IM: Those viruses are designed to steal login and password information of instant messaging programs.
Trojan-Ransom: Those Trojans are also known as “Ransomware” and are designed to disable use of the computer. Once the computer is infected the cyber criminals will demand a ransom, often by hiding behind legitimate authorities logos such as the FBI, in order to recover control of the infected PC. Ransomware can either block use of the PC or encrypt its files.
Trojan-SMS: These threats are designed to send SMS messages from your number without your consent. This can lead to illegitimate costs from your mobile company.
Trojan-Spy: Those viruses are designed to spy on your computer activity. It can monitor key strokes and take screen shots that are then sent to the remote hacker for criminal purposes.
Trojan-Mailfinder: Those threats are used to collect email addresses found on your computer in order to send spam messages.
Trojan Droppers: Droppers are used by hackers to drop viruses in a similar way as downloaders. Those threats will lead to further malware infections. Droppers are especially effective at hiding from antivirus programs.
A computer virus is a program designed to self-replicate upon execution. Most viruses are spread via executable files and require some sort of action to be activated such as opening an email attachment or opening an unknown file.
Downloads is probably the most used virus spreading method, especially on torrent exchange clients. They can also be bundled with other software that are offered for free.
Computer viruses can replicate in programs, files or sectors in the infected computer’s hard drives. Viruses will corrupt files and cause other types of issues on the infected PC.
Worms do not affect files, they are primarily designed to spread from one computer to the other using computer networks. Worms locate vulnerabilities on networks and exploit them to spread effectively, this happens mostly on non-updated operating systems.
While works usually do not spy on personal data, it can carry other threats, in such cases the worm is nothing but the vehicle transporting the threats to multiple computers.
Spyware is a malware category that allows a remote user to covertly analyse and collect data found on the infected computer.
Spyware is very hard to detect as it is usually only activated when used by the remote hacker. When the spyware is activated, its user can potentially steal login, banking or other type of information.
Spyware can also be used to spy on your activities in order to target you with specific advertising based on what you visit online.
The most common type of spyware is keyloggers. Those malicious programs are designed to spy on key strokes, they will report what was typed and where it was typed (on which URL) therefore giving away the login information to the hacker. Some keyloggers can also take screenshots of the infected computer to go around some protection methods that can be use to protect password information.
PUP stands for Potentially Unwanted Program. Those kind of software are usually installed with user consent along with free software, they are in fact the way companies have found to monetize users who are not willing to pay for their products or services. The PUP is bundled in the free software installer and the user downloading the free tool agrees to install the PUP in exchange for the free product, or it must be installed to access a free online service.
PUP are primarily designed to generate advertising revenues, they may modify browser settings (commonly known as browser hijackers) so that users eventually click ads on the unwanted search engine to generate an income. PUP can also be adware programs which are designed to show a certain amount of ads to the user everyday in exchange for the free tool or service.
PUP are said to be "potentially unwanted" because most people do not read the terms when they download a free software or subscribe to a free service website they legally agree by clicking "I agree" or "Next" but since they did not read the terms they are unaware of what they are agreeing to.