Viruses: Software that can replicate itself and spread to other computers or are programmed to damage a computer by deleting files, reformatting the hard disk, or using up computer memory. It is contagious and often spreads through software shared between computers. They can be subdivided into; file viruses, script viruses, macro viruses and boot sector viruses.

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Spyware: Software that secretly gathers private information about the user's behaviours and transmits it to interested parties. With the use of keyloggers, they can steal passwords by detecting keyboard inputs, which is why banks often use dropdown boxes for inputting sensitive data. Other types of information that is gathered includes the Web sites visited, browser and system information. It's nearly all done for illegitimate financial gain.

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Adware: Software that is financially supported (or financially supports another program) by displaying ads when you're connected to the Internet and open your browser. Not all Adware is malicious or illegal, but either way, it slows down your computer, can be extremely annoying, and should be removed.

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Worms: Unlike viruses, worms are malicious programs in them selves and can propagate on there own without a host program. Once a worm has found it's way into the system, it can replicate it's self many times and spread through whole networks, in some cases, causing serious damage. The financial implications can be catastrophic. Worms often enter a computer via an email attachment, following a link sent in an email or from peer-to-peer file sharing.

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Bots: Often entering a computer after the user has inadvertently downloaded the infection via email phishing messages that urge the recipient to click on a link or a social media message. The purpose of malware bots is to gain control over a computer and yield specific information like passwords, banking details and other personal data. They are self-propagating and geared to infecting large numbers of computers which then link to the head quarters of the cyber-criminals. The victim's computer can become part of a Botnet, which is a network of infected computers.

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Trojans: There are many types of trojans out there and they generally enter a system disguised as legitimate software, often because the user has been tricked into installing them. The primary function of a Trojan is to allow cyber-criminals to gain access to the user's operating system in order to steal information and data. These malicious tricks can sometimes involve modifying, copying and deleting data - even blocking data access. Probably the worst type of Trojan is Ransomware - read on..

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Ransomware: This type of malware is becoming increasingly common and more sophisticated. Basically, once infected, the user's machine is held hostage until a ransom fee is paid to the cyber-criminal. I like to split them into two different categories:

1. Lockout - this type interrupts the users' computer experience, either by covering their desktop with an image that the user can't remove, or by locking the user out of their system, often by using 'Syskey' (built into Windows) locked with a password only the cyber-criminal knows.

2. Crypto-ransomware - Once this ransomware has taken hostage of the computer, it will search through hard drives, network drives (including cloud storage folders) and any connected media (such as pen-drives), for files to encrypt. The attackers use a method called asymmetric encryption, which depends on two keys, a private and a public. The data is encrypted using the public key which can only be decrypted with hacker's private key.

Which ever type was used, the attacker will demand payment from the user, in return for putting the system right. Evidence suggests they usually can't be trusted to keep to their side of the bargain, whether by incompetence or malice! If you are further interested, you may want to read here.

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General

It should be noted that the above is not an exhaustive list as the variety of malware out there is continuously on the increase.