Some web sites need to store information on your computer. Why?
Let's imagine that you sign in to Amazon.com by entering your Amazon.com username and password. From now on, Amazon.com knows who you are. If you close your browser, and after an hour you enter the Amazon.com site again, Amazon.com still knows who you are. How does it know?
It knows because, when you sign in or log in to Amazon.com, the Amazon.com web site places a cookie in your computer. The cookie contains information like:
So - every time you connect to the Amazon.com web site, the cookie that Amazon placed on your computer is sent back to Amazon. This is how Amazon.com knows who you are, and what you have in your shopping cart.
Let's say you use the Google search. On the Google web site, you click on Preferences, and set some preferences - for example, you ask Google to show you 50 search results per page instead of the default 10 search results per page. After this, every time you do a Google search, Google will show you 50 search results per page instead of the usual 10. How does Google know that you selected 50 search results per page?
It's simple - it stores the preferences you selected in a cookie on your computer.
Every time you visit Google, your web browser sends back the cookie contents to Google, and so Google knows what preferences you have set.
Which cookies you decide to keep is up to you. For example, if you go to Amazon.com often, and want to login automatically into Amazon.com, then don't delete the Amazon.com cookie. If you see cookies from sites you didn't visit, or visited very rarely, you may wish to delete those cookies.