Service Applications(SA) are software services which run in a SharePoint farm. These are very useful to share information, resources and capabilities across multiple sites/applications/servers in the same share point farm or even across farms.
Service applications are introduced from SharePoint 2010. Advantage with the Service Applications are salable and extensible.
Sample Service Applications:
Access Services
User Profile Service
User Profile Synchronization Service
SharePoint Server Search
...
Location of services : Central Administration -> System Settings -> Manage services on server