What is a cookie?
A cookie is simply a small text file that is sent from a website to your computer (or mobile device) when you visit certain web pages. In its most basic form a cookie just stores the name of the website you visited, and a unique user ID. It cannot be used to identify you personally.
Then when you visit a website, your computer checks to see if it has a cookie for that site, and if it does, it sends information back to the website. The website then 'knows' that you have visited before, and in some cases can tailor the content to take account of that fact.
For example, you might visit a site and decide you want to view it with a larger text size. The next time you visit, if this information has been stored in a cookie, the website will 'know' your preferences and automatically set the text to a larger size.