Javascript Cookies
Cookies are used to track when people have visited your website. When you visit a website they will send you a cookie and might ask for something like your name. The cookie is stored on your machine and the next time you visit that site it will retrieve the cookie on your machine and it will be able to say "Welcome back your name!"
Read the following page and look at the examples for more information: http://www.w3schools.com/js/js_cookies.asp
For the assignment you need to explain what a cookie is and why they are used. It would be excellent if you put the time in to produce your own example.