The Information Commissioner’s website has a page of guidance about cookies.
For more about cookies, see www.allaboutcookies.org/ , which will also tell you how to disable cookies if you don’t want this site (or other sites you visit) to use them.
If you want to read more about The Privacy and Electronic Communications (EC Directive) (Amendment) Regulations 2011, the ICO has a page of useful information about PECR.
Suppose you decided you didn’t want this site to place cookies on your machine.
To save asking you every time you visit the site, the obvious approach would be to remember your preference using a cookie.
But that would now be illegal.