The Public Suffix List is a catalog of certain Internet domain name suffixes. A "public suffix" is also known by the older term effective top-level domain (eTLD). The Mozilla Foundation maintains suffix list for the security and privacy policies of its Firefox web browser, though it is available for other uses under the Mozilla Public License (MPL).
The List is used by Mozilla browsers (Firefox), by Google in Chrome and Chromium projects and by Opera. Future versions of Internet Explorer will also use the list.
According to Mozilla,
A "public suffix" is one under which Internet users can directly register names. Some examples of public suffixes are ".com", ".co.uk" and "pvt.k12.wy.us".
While ".com", ".uk" and ".us" are top-level domains (TLDs), Internet users cannot always register the next level of domain, such as "co.uk" or "wy.us", since these may be controlled by domain registrars. By contrast, users can register second level domains for ".com", such as "example.com", since registrars control only the top level of ".com". The Public Suffix List is intended to enumerate all domain suffixes controlled by registrars.
Some uses for the list are: