What are cookies?
Cookies are small, usually randomly encoded, text files that help your browser navigate through a website. The cookie file is generated by the site you're browsing and is accepted and processed by your computer's browser software. The cookie file is stored in your browser's local folder or subfolder.
Your browser accesses the cookie file again when you visit the website that created the cookie file. The browser uses the information stored in the cookie file to help ease your navigation of the website by letting you sign in automatically or remembering settings you selected during your earlier visits to the website, among many other functions.
Any particular website cannot access information on your computer other than its cookie. The cookie is not executable code so it doesn't have any "life" of its own, other than being used by the website that created it. A cookies use is limited to helping your browser process the information located on the website.