To minimise the impact of spam on web users, Google has decided to refresh their definition of "doorway" pages in an attempt to prove the quality of the user's search experience.
Websites maximise their search-ability with the use of doorway pages, however this often comes in the form of spam. Google is now keen to optimise these doorways (especially on larger sites) by routing the original site onto other sites and pages that are actually relevant to the original site searched for; ensuring doorway pages are relative in content and purpose to the page they have come from.
How could this impact your website or the sites that you visit? Google's official news page explains all with an update on doorway pages.