A web service has two components:

• A web-accessible interface provided by software that runs on a server; this interface performs operations in response to requests issued over the World Wide Web
• A protocol for issuing requests over the World Wide Web;in response to these requests, web services access appropriately configured web-accessible resources on the server. Web service syntax defines required input parameters, operation output, and the results of any operations performed.

In addition, the term web service is often applied to data hosted using an application that provides web services.

Web services facilitate interoperability by allowing the client and the server to develop independently. This means that regardless of client or server make and model, regardless of changes to content on the server, a web service will be able to respond to requests as long as those requests are made using correct syntax.

The Open Geospatial Consortium has produced several different flavors of web service that are relevant to geographic information systems, USGIN, the National Geothermal Data System (NGDS), and the AASG Geothermal Data project. These include:

• Catalog Service – Web (CSW): Catalog services are designed to query databases containing metadata about other services, thereby allowing users to discover and access services more easily
• Vocabulary Service: Vocabulary services are designed to systematically provide web-accessible definitions for specific vocabulary terms
• Web Coverage Service (WCS): Web coverage services are designed to publish continuous data sets. Continuous data sets are so named because the data they display does not have discrete boundaries. Consequently, continuous data sets are unsuited to vector images and are instead stored as raster images.
  
  For example, data about oceanic temperature is a continuous data set, since it is difficult to establish clear boundaries in a body of water. Consequently, data about oceanic temperature would be stored as a raster image and published as a web coverage service.
• Web Feature Service (WFS): Web feature services provide georeferenced features described by attributes.
  
  For example a feature service containing data about river systems might provide features with linear geometry representing river segments; each feature might be described by attributes such as average flow rate, width, and depth for the segment.
  
  Feature services are useful for geospatial analysis. For example, a client application used to calculate the most efficient route between points in a city utilizing real-time traffic information would require a representation of the streets as features described by attributes.
  
  Owing to the large amount of data associated with WFS, these web services tend to require fast Internet connections.
• Web Map Service (WMS): Web map services provide a georeferenced map image within a geographic bounding box; they typically provide georeferenced rasters of vector-based features. Web map services are most useful for visual exploration of geographic relationships.
  
  Web map services respond to getMap requests by returning an image file (typically *.tif, *.jpg, *.bmp or *.png) for the requested area. GetMap requests specify the geographic area of the bounding box and the map that is desired; a variety of other parameters allow control over image size, map projection, and other details.
  
  For example, a getMap request could be used to request a georeferenced satellite photo of Arizona; if the satellite photo exists on the server as a web-accessible image file, the web service will respond to the getMap request by providing the client with the desired image.
  
  Web map service image files are often based on a source shapefile or feature class; consequently, web map services support FeatureInfo requests at point locations. FeatureInfo request returns are not standardized and depend on server configuration.