A cookie is a small text file that is sent to your computer via your web browser when you visit some websites.
These cookies are used to store information about you for the next time you visit that site - information like where you went on the site and what you did.
Cookie files are a great way for a website to remember you when you next visit, but if you want to surf the net without worrying about your privacy or security being threatened, you might want to stop them being sent.
What they do
Cookies have a variety of different functions, from remembering purchasing information or recording your viewing preferences, to storing scores for online quizzes.
All of which can be useful if you repeatedly visit the same sites time and again.
A cookie from a particular website will sit on the hard drive of your computer and wait until the next time you visit that site.
Without the cookies the web server would have no way of knowing that you had been to the site before unless you logged in with a username and password.
Cookies allow web servers to remember you.
How cookies work
Let's say you go to a website and it has a quiz with question one on the first page and question two on the second etc.
When you go to the second page, the web server needs some way of knowing that you are the same person who answered 'A' for example on page one.
That way, when it comes to adding up your score it can combine all your answers, from many different pages, all together.
Putting a cookie on your computer allows the server to remember that you are the person with that unique cookie and it can recognise you wherever you go on that site even though you aren't logged in.
If the computer didn't give you a cookie it wouldn't know whether it was the same person visiting all the pages.
It's because of things like this that some websites even refuse to let you in if you don't accept cookies - so you could be missing out.
Third party cookies
Third party cookies are the ones that have got users concerned about their privacy most worried.
In theory, only the website which placed a cookie on your computer can see that cookie so it shouldn't be possible for any other sites to look at that cookie.
When a cookie is placed on your computer, it records the name of the website that will be allowed to reopen it in the future.
If 'Super Supermarket' places a cookie in its name then only 'Super Supermarket' should be able to open it.
By doing this marketing companies can track you across all the sites that use their cookies.
They do this to get an idea of your viewing and shopping habits and then share that information with the websites who are their customers.
So, let's say you buy nappies at 'Super Supermarket' other websites can start trying to sell you other baby products.
If you give your e-mail address or full name to one site it could share them with other marketing partners, and you could even start to end up with spam for baby products in your e-mail inbox!
Can they damage my computer?
Despite their reputation, cookies are very small files that don't cause damage to your computer just by sitting on your hard drive.
In general, cookies are just text files and are not executables so they cannot spread viruses around your computer.
However, as well as third-party cookies they do also contain information such as user names that could reveal your identity.
To view the web anonymously, you could consider getting rid of your cookies. The easiest way to do this is to refuse to accept, or 'disable' them in your web browser.
However, this can be a bit limiting, because many features in sites won't work if you disable cookies.
Because some sites prevent you visiting them without a cookie, most web-savvy internet users simply delete their cookies every so often.
This may sound complicated, but deleting your cookies is actually relatively straight-forward.
In Internet Explorer, simply go to 'Tools', 'Internet Options' and then click the 'Delete Cookies' option.
Organising your cookies
In most web browsers there is a function for organising your cookies and setting the level of security so that you only receive cookies from certain safe sites.
In Internet Explorer 6, select 'Tools' then 'Internet Options' and then 'Privacy' - this will open a window with various different options.
You can use the slider to change which cookies you are willing to accept and use advanced settings to block or accept cookies from specific sites.
Other versions of Internet Explorer, Netscape and other browsers have similar settings.