A CDN is a specialist network of computers optimised to deliver web sites, pictures, videos and software downloads to people visiting a web site in the quickest, most efficient way possible. Computers in the network work together to share the load of delivering these files and to enable companies to deal with large peaks in demand.

CDN providers include companies such as Akimai, Coral CDN and Limelight Networks.

When delivering a web page from a web server to a user's machine, distance is a factor. The distance from a user's machine to the actual location where the files are hosted does have an impact on download speeds. Use of a CDN can reduce download delays by detecting country the user is located in and using files from the geographically nearest web server.

A very successful company offering software or documents for download might go beyond a regular hosting arrangement and employ the services of a CDN so that users in New York, Amsterdam, Buenos Airies, Cape Town, Shanghai and Melbourne all receive a similar service when downloading their products.

So who would use a CDN?

Companies that have websites with a massive readership or a very large dispersed customer base could benefit from using a CDN. In these circumstances it is very likely that their users will be dispersed globally and the performance gain of locally cached resources will improve the speed of delivery for the use.

But CDNs aren't just for large companies. For example, people often embed shared assets such as video, widgets or JavaScript libraries in their web sites and these can be hosted on CDNs.

When we sign up for services online, or log in to secure pages we are usually asked to provide a username and password. In order to disincentivise people writing software to sign up to things in bulk you are sometimes shown a picture of some letters which have been distorted in some way. This method of stopping fraudulent sign ups and spam is called a CAPTCHA.

CAPTCHA stands for Completely Automated Public Turing test to tell Computers and Humans Apart and is devised as a way of trying to ensure that the person signing up for a service or sending an enquiry via an online for is indeed a person and not an automated programme used by hackers.

Alan Turing (the T in CAPTCHA) was a pioneering British computer scientist why as well as working at Bletchley Park helping to crack the Enigma code, also devised a series of tests to determine whether tasks had been performed by a human or a computer.

In the ReCaptcha system the user types two distorted words. The words come from scans of books that are being digitised as part of the Google Books project. In this video Luis von Ahn explains the system.

The cloud is a very generic term used to mean the internet and is becoming more prevelant as business and leisure activity moves to the internet.

Cloud is most commonly used as a prefix for terms to indicate that they are carried out on the internet, not on an individuals computer. Cloud storage involves storing your files online and web based email services such as Yahoo Mail and Gmail are a form of cloud storage.

At present the most common way to work on a computer is to used desktop programmes such as Microsoft Office or Open Office to create reports, letters and spreadsheets. We store the files, photogrpahs and emails that we create on our own computer or on a server at work. This model is changing towards more emphasis on the cloud.

The cloud is becoming more popular as people want to be able to access their data from a variety of places and via a number of different devices.

It is thought that the term originated because the internet is usually depicted as a cloud symbol in network drawings.

One feature of blogs is that reader of the blog can usually leave comments about each article. When leaving acomment, the reader will be asked if they have a website. If the reader give their website address, other readers can them click on that persons name to see their website.

Links from other websites and blogs to your own website can be benificial for promoting your own website and for improving your search engine ranking.

Some people leave comments on website that have no relevance to the article simply to try and get a better search engine ranking.

However, most modern blog software add a special code to the link to tell search engines not to follow the link provided by the commenter. This means that the link is worthless in terms of improving the commenters search engine ranking. This special code is inlcuded to try to discourage people leaving comments simply to improve their search engine ranking.

Another form of comment spam is to put links to other web sites solely for the purpose of advertising a commercial service rather than contributing something worthwhile to the article. These types of comments are often added to blogs by software rahter then someone actually visiting the blog.

Providing a link in a comment does not mean that you are a spammer, but your comments should be relevant to the article and add something to the debate.

The amount of comment spam and other abuses of the commenting system in blogs has forced some people to switch off comments in the blogs or to set up their blog so that all comments have to be moderated.

Server Co-location (or Co-Lo)  is the process of placing computer equipment that you own in a secure web hosting facility.

All of the files for a web site are stored on a computer called a web server. These web servers are kept in a special facility called a data centre which has very high capacity connections to the internet and specialist power and cooling equipment.

The very vast majority of website are run on shared hosting accounts which means that one web server may run several thousand websites. But when a web site becomes busy enough, or if a very high level of security is required, it needs to be hosted on a dedicated server. In other words, a web server dedicated entirely to that one web site.

When an organisation has to go down this route, they have the option to either lease a dedicated server from a hosting company or co-locate their own equipment. A co-location contract covers the provision of space in a data centre for your equipment and the connection of this computer to the internet.

See 1U for more information about hosting racks.

CMS is an acronym of  Content Management System and is widely used to describe software that is put in place to enable people to update and manage their own websites.

A CMS usually stores the content used on web pages in an online database which can then be added to and edited by members of the organisation as part of the publishing process.

Different people in an organisation can be assigned different levels of access to a CMS. E.g. some people could be given the ability of adding new pages whereas others may only be able to edit or approve changes to existing pages.

Many off the shelf software CMS packages exist or you can ask a web development company to create a bespoke package to suit your specific needs or business sector.

Poor old cookies. They have a undeservedly bad reputation, but without them the web would be a much more annoying place.

A cookies is simply a small amount of text data that a website can save on to your computer. It can store basic information such a membership number.

This is how so many websites are able to "remember" who you are when you go back. This means that you don't have to fill in all your details each time you want to do somthing on a site like make a purchase.

A website can only access the cookies that it has created. It cannot access cookies from another website. E.g. Amazon.com cannot access cookies left on your computer by Apple.com

Many people seem to think that cookies are harmful to their machine, that thay can carry a virus or control programmes on their computer. They are only text files and cannot do anything to damage your computer.

Many web sites and blogs carry advertising. Advertising brokers such as Google's AdSense make it easy for anyone who runs a web site to add advertising to their web site without having to build individual relationships with advertisers.

The advertising is known as contextual because a small piece of code is placed on each page of the web site and whenever the page is shown, advertisements which are relevant to the content of the page are displayed. This is seemingly magical process is achieved by the advertising broker using software to analyse the subject matter of a page and match up a relevant advertisement.

Web site owners that display the advertisements usually earn money when a visitor to the web site clicks on one of the advertiements and is taken through to the advertisers web site. Other advertising brokers may pay web site owners based on the number of times the advertisement is viewed.

Servers in a data centre are kept in a cabinet. Each cabinet usually has the capacity to hold in the region of 48 servers.

Cabinets aren't just about keeping things neat and tidy. Each cabinet  provides the infrastructure required for cabling to connect the servers to the internet, electrical power, cooling and heat distribution and security.

Each cabinet in a data centre can contain enough web servers to run hundred of thousands of web sites.

A class is an attribute that is applied by a web designer to a specific set of elements in a web page. The class has instructions about how to display those elements when the page is loaded.

Classes are often defined in a seperate file called a style sheet which can be used on all of the pages that make up a web site.

For example, if we have a calender grid displayed on our page, we may apply a class called "weekend" to the columns in the calendar that are for Saturday and Sunday.

In the style sheet for the web site, the class "weekend" could contain instructions to highlight the columns in a bright colour such as yellow and make the letters bold.

Classes don't have to exists in a separate fille called a style sheet, but by keeping all of the classes in one place (a separate file) presentational information is much easier to maintain and change.