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.

ADSL or Asymmetric Digital Subscriber Line is what is usually called broadband. It is much faster than a traditional modem over a telephone line because it uses "spare capacity" on the telephone line. The asymmetric aspect of it is that upload and download speeds are different. Because the majority of our interent usage involves downloading information, the download channel of ADSL is much faster than the upload channel.

Wi-Max or Worldwide Interoperability for Microwave Access, is a wireless technology that enables computers and mobile devices to connect to the internet.

It is like the Wi-fi connections that many people have a home or can access in cafes and airports. The main difference from the user's perspective though is that whereas wi-fi has a range which can be measured in 10s of metres, wi-max range is measured in kilometres. Typically Wi-Max networks range from 3-10 kilometres and do not rely on "line of sight".

Wi-Max is based upon the IEEE 802.16 international standard and at present has transfer speeds comparable to domestic ADSL, upwards of  1-5 Mbps.

This is a web site hosting term that describes the amount of data that can be transferred to and from your web site. Most web hosting companies provide you with a monthly bandwidth allowance.

Every time someone visits your website, they will download all of the files required to display the page on their computer. The amount of data that has to be sent between your web site and the visitors computer is added to the amount of bandwidth you have used that month. Likewise if someone downloads a document from your web site such as a flyer or an application form the amount of data transferred is also added to your monthly total.

If the amount of data that you transfer exceeds your monthly allowance your hosting company will probably charge you an addition fee for each megabyte of information transferred.

All web sites have ongoing costs that they have to take into consideration. One of these is an annual or monthly charge for web hosting.

Web hosting charges can usually be broken down into storage space, bandwidth allowance and services such as databases.

The bandwidth allowance is the amount of data that you are allowed to transfer each month from your web site to you visitors computers. Each page on your web site may be about 20 Kilobytes and if you have 10,000 page views per month then you will transfer approximately 200 Megabytes of data.

Bandwidth theft is where another web site displays a file from your web site on their own web site. For example, if you have a nice photograph on your web site  that is 20 Kilobytes and another web site uses it on one of their web pages by loading the photograph from your server each time their page is displayed, this is bandwidth theft.

In this case they should either ask for your permission to download the photo and host it themselves or link to your page with the photograph rather than displaying it.