Also known as browser cookies or tracking cookies, cookies are small, often encrypted text files, located in browser directories. They are used by web developers to help users navigate their websites efficiently and perform certain functions. Due to their core role of enhancing/enabling usability or site processes, disabling cookies may prevent users from using certain websites.
Cookies are created when a user's browser loads a particular website. The website sends information to the browser which then creates a text file. Every time the user goes back to the same website, the browser retrieves and sends this file to the website's server. Computer Cookies are created not just by the website the user is browsing but also by other websites that run ads, widgets, or other elements on the page being loaded. These cookies regulate how the ads appear or how the widgets and other elements function on the page.For Managing cookies for different browsers see here
Website servers set cookies to help authenticate the user if the user logs in to a secure area of the website. Login information is stored in a cookie so the user can enter and leave the website without having to re-enter the same authentication information over and over. More information
Session Cookies are also used by the server to store information about user page activities so users can easily pick up where they left off on the server's pages. By default, web pages really don't have any 'memory'. Cookies tell the server what pages to show the user so the user doesn't have to remember or start navigating the site all over again. Cookies act as a sort of “bookmark” within the site. Similarly, cookies can store ordering information needed to make shopping carts work instead of forcing the user to remember all the items the user put in the shopping cart.
Persistent or tracking Cookies are also employed to store user preferences. Many websites allow the user to customize how information is presented through site layouts or themes. These changes make the site easier to navigate and/or lets user leave a part of the user's “personality” at the site. For Information on session and persistent and tracking cookies, see here
Cookies are NOT viruses. Cookies use a plain text format. They are not compiled pieces of code so they cannot be executed nor are they self-executing. Accordingly, they cannot make copies of themselves and spread to other networks to execute and replicate again. Since they cannot perform these functions, they fall outside the standard virus definition.
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. Many anti-spyware products are well aware of this problem and routinely flag cookies as candidates for deletion after standard virus and/or spyware scans.See here for some privacy issues and concerns.
Most browsers have built in privacy settings that provide differing levels of cookie acceptance, expiration time, and disposal after a user has visited a particular site. Backing up your computer can give you the peace of mind that your files are safe.