ArcGIS for Server, ArcGIS Online, Portal for ArcGIS deliver GIS capabilities in a services-based architecture. GIS maps, features, imagery, geoprocessing models, locators, and datasets are deployed as web services, which can be accessed by essentially any web client or device. GIS services can be deployed and scaled using banks of computers, both on-site in your organization and in cloud servers on the web. ArcGIS Online is an example of a cloud system offering "GIS-as-a-service" capabilities.
Users create important GIS maps and information using ArcGIS for Desktop and publish these as web services using ArcGIS for Server and ArcGIS Online. In this way, ArcGIS provides broad access to information and the ability to scale your GIS to fit any configuration or situation.
ArcGIS provides an open web API for the sharing of map and GIS services that follow RESTful design patterns. These maximize opportunities for adoption and use. This enables developers and application builders of all kinds to combine and mash up ArcGIS services with any other web information coming from many different sources. It also gives consumers access to the tremendous wealth of geographic knowledge created in GIS environments.
Consider using services if: