SharePoint 2013 has a much improved and powerful search platform. FAST search is integrated with SharePoint in 2013.
SharePoint 2013 search gives us the ability to customize how search results are queried and displayed and also the ability to select external sources. In 2010, we often use Search Scopes to define the subset of data. Users select the scope which is displayed as a drop down right next to search box, to filter or target a defenite subset of results.
In SharePoint 2013, Search Scopes are no more. They are combined with federated search functionality and resulted in new concept called Result sources.
We can accomplish more using Result sources than search scopes in 2010. Result sources can be created at Search Service Application level, site collection level or site level. Search settings performed at service application level are available for whole farm, site collection level and available for sites under that site collection and at site level are available only for that site.
SharePoint 2013 gives greater ability to manage search at different levels.
Apart from these 4 displayed, there are in total 16 Result sources which are defined by default. They can be accessed through Central Admin -> Application Management -> Manage service applications -> Search service application -> Result sources under query results.
Any New Result sources created here are available across the farm. New Result sources can be defined with the help of query builder available in 2013. Once a result source is created at this level, it can be consumed at any site collection or site level.