Cookies - what are they good for?

[#1]

TV programmes such as Click refer to cookies but I've never seen details what exactly they can do, for example if company A places a cookie on someones computer what's to stop company B accessing the same cookie and using it?
Information about what cookies can do seems rather vague.

[#2]

Cookies are small text files that store pieces of information used by the website to control its behaviour and to store preferences etc.
If the standard security is working,  the information is only available to the site that sets them.
Fundamentally, ther are two types of cookies.
Session cookies - these are temporary cookie files, which are erased when you close your browser.
Persistent cookies – these files stay in one of your browser's subfolders until they're deleted  (manually or expired)
This site should  help...  http://www.allaboutcookies.org/

[#3]

Thanks for the response, from the website...

Quote

Cookies CAN be used for malicious purposes though. Since they store information about a user's browsing preferences and history, both on a specific site and browsing among several sites, cookies can be used to act as a form of spyware.

On the one hand we are told cookies are harmless then we are told the above, how can it be spyware?

[#4]

To take a simple example of using cookie data; website developers use them as a temporary store of data about the remote person's computer and sometimes any personal data which may have been entered. If you revisit the same site a few days later the server can check to see if there are any  cookies for you from previous visits. The data on the old cookie can be used to "improve your experience", it also helps to provide a data trail for the website owner. The problem is when other malicious website developers actively search out old cookies on your PC and then try to misuse that data.
For  example a poorly designed application might hold your name, password, date of birth, street address and postcode in the cookie on your PC. Such information might be extremely attractive to the bad guys.
Other developers might check which cookies are present to see which web sites you have visited, e.g. BBC News, Demon Internet, Leather Sadomasochism, knitting patterns.

[#5]

Just to raise the paranoia level a bit - don't forget so called flash cookies

[#6]

Controlled here http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager07.html

[#7]

For dealing with trackers,  on most browsers Ghostery is useful

[#8]

A simple tip is to install another browser to be used specifically for things like online banking to keep those important personal details away from your general browsing.
For anyone tempted to browse the seedier side of the internet, use a live linux CD or similar for this purpose.

[#9]

Quote from: christmas

Thanks for the response, from the website... Quote Cookies CAN be used for malicious purposes though. Since they store information about a user's browsing preferences and history, both on a specific site and browsing among several sites, cookies can be used to act as a form of spyware. On the one hand we are told cookies are harmless then we are told the above, how can it be spyware?

Cookies themselves are NOT spyware. They are simply text files that hold pieces of information. Most browsers will only make the cookies available to the website domain that set them.
EG if hotmail.com set a cookie on your computer (by sending a command in the webpage headers that instructs the browser to set a cookie) then it will only be readable by hotmail.com (the browser will automatically send that cookie back with each request to the domain it matches - in this case it will look for any cookies set by hotmail.com and send them back).
Cookies are not spyware. They hold information only such as user preferences, user is to stay logged in (eg this forum has a "Keep me logged in" feature which no doubt uses a cookie), page colour preferences etc.
The risk from cookies comes from websites that use clever techniques such as javascript / ajax to read cookies OR from external software that you may download that may be able to access them and read them. These techniques are not blocked by the browsers built-in rules that cookies only apply to the site that sets them and thus that means that they can be accessed by these other means.
The cookies themselves are not pro-active spyware, they simply hold information. The paranoia brigade kicked up a stink about them years ago and ever since then they've been in the news every few months being claimed as a risk when in fact there is little risk from them.