ArcGIS Server is a comprehensive platform for delivering enterprise GIS applications that are centrally managed and support multiple users. ArcGIS Server provides the framework to build and deploy centralized GIS applications and services to meet a variety of needs using a variety of clients.

ArcGIS Server is a platform for delivering focused enterprise GIS applications that are centrally managed, support multiple users, include advanced GIS functionality, and are built using industry standards. These applications can be accessed by a range of clients, from thin (browser-based) clients to mobile devices to thick desktop clients.

There are three products in the ArcGIS for Server collection; each adds a higher level of functionality.

• ArcGIS for Server Basic
• ArcGIS for Server Standard
• ArcGIS for Server Advanced

ArcGIS for Server key features:

• Deliver GIS Services
• Respond to the Demand for Maps and GIS Tools
• Power Applications with GIS Capabilities
• Start Using GIS Services Right Away, No Programming Necessary
• Manage Enterprise Geodatabases

For more information on key features visit: http://www.esri.com/software/arcgis/arcgisserver/features

New capabilities can be added to server products through a series of ArcGIS extensions:

• ArcGIS 3D Analyst for Server
• ArcGIS Spatial Analyst for Server
• ArcGIS Spatial Analyst for Server
• ArcGIS Network Analyst for Server
• ArcGIS Image Extension for Server
• ArcGIS Data Interoperability for Server
• ArcGIS Workflow Manager for Server
• ArcGIS Schematics for Server

Detailed descriptions for the ArcGIS for Server extensions are available here.