The EU e-Privacy Directive, otherwise known as the EU Cookie Law, originally came into place on the 26th of May 2012 and means you, as the site owner/administrator, have to get your visitors' informed consent before placing a cookie (probably related to social media elements or login/tracking systems) on their machine.
If you are a WordPress admin, there is a notification plugin for you to use which looks like this:
There is also a great overview.
According to the above link, it is not just standard cookies.
The law also affects anything that acts like a cookie, for example:
Flash Cookies and HTML5 Local Storage.
There are also 'suggested categories' from the same link above (overview):
- Essential (logins)
- Non-Essential but harmless (functionality, not essential)
- Fairly Intrusive (web tracking)
- Very Intrusive (PII - personally identifiable information)