Source: http://blogs.msdn.com/b/russmax/archive/2013/06/06/search-2013-search-architecture-part-1-intro.aspx
Several things have changed in SharePoint 2013 Search. As you might already be aware, we merged and built upon the core SharePoint 2010 Search product and Fast Search for SharePoint 2010 into a single product\engine simply called SharePoint 2013 search. This is the first of a series of Search 2013 Architecture blogs. So much has changed that it requires a series of different blog posts covering all of the areas. This blog will just highlight the major changes and provide a quick intro into Search Architecture. The remaining Search Architecture blogs will dive into greater detail on aspects of Crawling, Feeding, Indexing, Fetching, and Search Administration.
Read More: http://blogs.msdn.com/b/russmax/archive/2013/06/06/search-2013-search-architecture-part-1-intro.aspx
Aryan Nava 