ArcGIS Server makes your data—such as features, tables, maps, tools, imagery, and locators—available to many other clients in your organization and, potentially, the entire Internet through web services.
Web services, essentially, are your data. They access the data in your storage locations, allowing you to share that data without giving people direct access to the storage locations. In the case of feature and imagery data, you can publish web services that access the data without duplicating it. If the data is relatively static, you can cache the data in a location separate from your data source to improve drawing performance.
The type of data you want to make available to others and what you want people to do with that data determine the type of web service you publish.
How ArcGIS Server stores and accesses data
Web services access the data from directories, databases, and geodatabases you make accessible to the ArcGIS Server site.
When you publish a resource such as a map, tool, or locator as a web service, that resource is placed on the ArcGIS Server machine. For example, if you publish a locator as a geocode service, a copy of the locator is placed on the ArcGIS Server machine to ensure that ArcGIS Server has sufficient permissions to access it. Similarly, maps you publish are copied to the ArcGIS Server machine. The ArcGIS Server site must have access to the location of the resource to copy it.
Some resources references additional data. For example, maps and geoprocessing models reference various feature classes stored in a database, geodatabase, or shapefiles in a folder. You need to grant ArcGIS Server permissions to the data in these locations and register these data sources with the ArcGIS Server site before you publish.
Functionality, web services, and data resources
In most cases, you publish web services to allow people to visualize and query your spatial data in a map format. The two main types of spatial data you share in this way are feature and imagery data.
To provide query access to feature data in a map format, publish an ArcGIS Server map service. The map service is the main building block for sharing feature data. The capabilities you enable on a map service provide additional functionality for interacting with the data. For example, to allow people to edit feature data or analyze feature patterns, publish a feature service, which is a map service with the feature access capability enabled. To allow people to work with utility features, publish a utility network service, which is a map service with the feature access, utility network, and version management capabilities enabled.
To share imagery (raster) data, publish an ArcGIS Server image service. These services can be used for reference in maps and apps or analyzed to reveal patterns in the images.
Other data resources you publish—such as geoprocessing tasks and locators—provide web-based GIS tools for analysis and geocoding, respectively.
To learn more about service capabilities, the resources you can publish, and the ArcGIS Server roles required to run each type of service, see What types of services can you publish?