Viruses
A virus is a general term for a range of threats which cause harm to data and computers, referrring to software that is designed to run on a computer system and replicate itself. Some viruses are harmless and are designed to annoy the user of the computer, but others are quite dangerous and can cause damage to data, and in some cases theft of private information such as passwords and banking details.

Unlike worms (see the next section), virii normally require some sort of user action in order to spread, such as someone taking an infected file to another computer on a memory stick, or saving an infected file onto a network folder. However more and more viruses are starting to take on similar characteristics to worms and therefore spread much more easily.

The best protection against a virus is a decent firewall as well as installing anti-virus software, and keeping these programs updated.

Worms
Worms are designed to replicate themselves quickly and spread through networks using file-sharing features found built-in to most operating systems and email clients. Although they do not always cause damage to data or computers, they utilize a lot of network bandwidth, which may cause disruption to users. Some worms also carry a ‘payload’ of code which may contain instructions to delete files, or make changes to a computer without a user’s consent. An example of a major worm is the ‘ILOVEYOU‘ virus from 2000 which caused an estimated $5.5billion worth of damage.

Trojan Horse
The name ‘trojan horse’ refers to a group of threats which are often used to create ‘backdoors’ in computer systems, which allow access to the infected system by an outside source. The name ‘trojan horse’ is a reference to the mythical story of the Trojan horse; the programs are designed to deceive the user into thinking it is a legitimate program (and sometimes it is a legitimate program, altered by a third party). Once it is downloaded and run, it may install backdoors or cause damage to files. Unlike a virus, trojan horses do not infect files; they are designed to deliver a payload to the system.

Adware
Adware, although not truly a "threat" by definition, often turns out to be a threat in how it is used. Adware refers to software designed to display advertisements to a computer user. It is often bundled with other software.

Some forms of adware are just an annoyance and will not interfere with computer usage too much, but some pop up regularly and may cause the computer to run slower or may pop up on top of programs in use. Some adware has inbuilt functions to track what a user is doing online; this information is then sent to the company, who may even sell it to someone else. Some adware informs the user it is doing this but on other occasions it will do it in the background without any warning (except sometimes the vendor might put a tiny note in the terms of use/disclaimer) This is where the lines between adware and spyware are blurred.

Spyware
Spyware is software designed to track what a user is doing on their computer. The data is then normally sent off to a company. However, spyware seems to be expanding beyond this, and it is often found to be dangerous or damaging to a system. Spyware has been known to hijack Internet browsers, display fake warning messages, and block websites, among other things.

Rogue Software
This is often considered to be related to spyware because of its method of getting onto a system. One of the most common ways that rogue software is installed on a computer is through a false error or warning message (from spyware) which says something along the lines of “Your computer is infected by spyware! To fix this, download <whatever>” These programs are either designed to make you buy a ‘full version’ of the software (which in the vast majority of cases does not really fix an infection) or it causes more harm to the computer.

A famous example of this is ‘Spy Sheriff’; it displays fake scan results, and when the user is browsing the Internet, it will often redirect them to adverts or random web searches. It also blocks several websites (including genuine anti-virus/spyware sites.) Rogue software is often difficult to remove and will normally require the user to remove it through Windows Safe mode (Spy Sheriff is known to block system restores, and even reinstalls itself if it is not removed fully).

Phishing
Phishing describes a technique often seen in emails by which a hacker will try to obtain private information. The most common example of this is emails claiming to be from a person’s bank, Paypal, or other financial services. Generally, the email will be designed to look like it is from the official website (e.g. Paypal) and it will normally mention that the administrators need to update your account information; some will ask for direct replies to the email and some will direct you to the website. However, the website they direct you to is not the genuine website, it is simply a form designed to send your details to the person’s email address so they can use your details.

Although the emails can be quite genuine-looking they should not be used; if you are going to visit the website mentioned, type it in yourself instead of following the links. Always remember that banks tend not to use emails for important notices - if they need your details they will more than likely send you a letter and ask you to go into your nearest branch.

Another example of phishing that I have seen recently is an email which gives the impression that it is from a solicitor or similar and claims something along the lines of “My client is a Russian oil tycoon who has been put in jail and is about to have his assets frozen, if you allow us to use your account to put the money in temporarily we will give you a 10% commission” These are ALWAYS complete scams, they are designed to get your account details and then the person will use them to take your money.

Pharming
Pharming is similar to phishing. The person will create a website designed to deceive the user into putting details into it (such as a PIN number or an account number), but instead of using phishing emails to get the user to visit the website, the person hacks the user's DNS server and changes the IP address of the real website to point at the IP address of the site they have set up. This gives the appearance that their site is a genuine one as in the address bar it will appear to be the genuine site.

This means that pharming can be much more convincing than phishing; unlike phishing it makes the new website address look exactly like the real thing, where normally a website which uses phishing to get people to go to it has a different address.

There are still other types of threats, but this information should give you some forewarning about the main types and help keep you safe.