About ArcGIS Server web services
An ArcGIS Server web service represents a GIS resource—such as a map, globe, locator, or geodatabase connection—that is located on an ArcGIS Server server and is made available to client applications.
Working with ArcGIS Server web services
You don't need any specialized GIS software to work with a service; you can consume the service within a web browser or custom application. However, ArcGIS applications, such as ArcMap and ArcGlobe, can also act as clients to GIS services.
When you work with a service hosted by ArcGIS Server, you have, in most cases, the same level of access to the resource that you would have if the resource were located on your machine. A map service, for example, allows client applications to access the contents of a map on the server in much the same way that they would if the map was stored locally.
Publishing a GIS resource as a service is the key to making that resource available to other people. As you deploy ArcGIS Server, you will follow the common pattern of creating resources in ArcGIS for Desktop and publishing the resources as services so that client applications can use them.
ArcGIS Online supports ArcGIS Server authentication for adding and accessing secured services. When you add a secured ArcGIS Server service to a web map, ArcGIS Online prompts you for credentials. It then takes care of obtaining a token for authentication; there is nothing additional you need to do. You should not attempt to append a token to the URL when you add a layer using the map viewer.
When you add a secured ArcGIS Server service that is exposed externally as an item in ArcGIS Online, you choose whether or not to have the credentials stored with the service item. If the credentials are not stored, ArcGIS Online will prompt you for the credentials every time you access the service. If the credentials are stored with the service item, you won't be prompted.
Types of services
There are two main types of ArcGIS Server services that can be used as layers for a web map: feature layers and map layers.
Feature layer
A feature layer is a collection of geographic features. Each feature in the collection has a location, set of properties, map symbology, and pop-up window. Feature layers allow you to execute queries on the features and perform live edits on the features using templates for an enhanced editing experience. A feature layer can be managed as a part of your content or referenced in a web map or application. The contents of some feature layers can be downloaded. Feature layers include feature services and hosted feature services.
Map layer
A map layer is a prerendered collection of map cartography organized by location and scale. Map layers can be dynamic or cached as tilesets.
Dynamic map layers generate map images when requested by the layer. As you navigate the map, new map images are displayed. Dynamic map layers include map services and image services. Image services are based on raster data which is essentially a grid of cells. Rasters are commonly used to store imagery and other information captured by satellite sensors.
A tileset is an organized collection of raster map caches combining terrain data, imagery, symbolized map features, and labels. Tilesets are created for specific geographic extents, projections, and levels of detail. They support fast visualization of complex maps using a collection of predrawn map tiles. These map layers are created and stored on the server after you upload your data. They are appropriate for basemaps that give your web maps geographic context. Tileset layers include hosted tiled map services.