In computer terms, a "cookie" is a small file containing information that is passed between a website and a person browsing the Internet. Cookies are an important part of the web surfing experience, and many of the features that we take for granted on websites would not work without them.

Definition

At its most basic level, a cookie contains a unique ID number that is given to you when you visit a website. The cookie is stored on your computer for a certain length of time--usually around 30 days--before being deleted automatically by the web browser. During that period, your browser will present the cookie to the website every time you visit, letting that website know that you have returned. Unless you have registered an account with that website, they have no personally identifiable information about you.

Purpose

Cookies allow website operators to easily track the areas of their websites in which visitors are most interested, which helps them deliver more of the content that their visitors want. Also, cookies make the experience of using a website more consistent for the visitor. They can prevent you from having to log in every time you visit a site, allow you to easily revisit the pages that you looked at on a previous visit, and so on.

Security Risks

Some websites may place a "tracking cookie" on your computer when you visit. Tracking cookies are considered by some to be an invasion of privacy, as they track your activity across multiple websites for the purpose of delivering targeted advertisements based on your Internet usage habits. Although none of the information collected can be traced back to you personally, it may be a good idea to periodically delete all of the cookies on your computer to remove tracking cookies. Free security software such as Ad-Aware can do this automatically.

Source:

What Is a Cookie?

Cookies: What They Are, Why You Are in Charge