In the previous lesson, you learned about some open formats for data stored on your machine as files and databases. Now, let's examine some of the open specifications for web services that draw maps.
What is a specification document?
Recall from Lesson 1 that a web service receives a user request and sends back a response. In order for web services to work across platforms, the syntax for the requests and responses needs to be consistent and openly described in what is called a "specification" document. When software developers create a server or a client that supports the web service, they treat the specification document as their Bible; it is a contract that must be closely honored in order for web services to correctly function.
To get a feel for what a specification looks like, take a minute to browse the Open Geospatial Consortium (OGC) WMS specification (especially Section 7). Don't worry too much about what this all means. We'll dive into some of that later in the lesson. Just notice that there are clearly defined methods and parameters that you and the server must use if you want to communicate with the service.
Because specifications must describe every method and parameter of the web service, they can appear to be long and complicated documents. Fortunately, you rarely have to use the specification directly because you will work with relatively user-friendly servers and clients are designed to abstract away the complexity of working with web services. For example, later in this lesson, you'll use GeoServer to create a WMS and QGIS to display the WMS on a map. You don't have to refer to the WMS specification in order to do this because these programs take care of formulating the request and working with the response. You can be certain, however, that the developers of GeoServer and QGIS had to examine the WMS specification in great detail when writing their source code.
OGC W*S specifications
The OGC is an industry consortium that develops open specifications for spatial data and web services. The OGC is comprised of representatives from private companies, government organizations, NGOs, and universities.
OGC has produced a series of specifications for GIS web services named in the format Web _____ Service (sometimes abbreviated as W*S). For example:
- Web Map Services (WMS) return a rasterized image of a map drawn by the server. This doesn't allow much complex analysis, but it is so useful for visualization that WMS has become the most widely implemented and supported of the W*S services.
The basic WMS draws the map on the fly, but additions to the WMS specification provide syntax for requesting predrawn image tiles from the server. To get started, we'll just work with the dynamically drawn WMS. - Web Feature Services (WFS) return vector geometries and attributes, allowing for spatial analysis and editing. The vector features are communicated in XML using Geography Markup Language (GML), itself an open specification defined by the OGC for transferring vector feature data.
- Web Coverage Services (WCS) return blocks of data called coverages (not to be confused with the old vector Arc/INFO coverages) that are commonly used for raster display and analysis.
- Web Processing Services (WPS) allow a user to invoke geoprocessing operations on the server.
Virtually all FOSS GIS servers and some proprietary ones have engineered their web services to communicate through OGC specifications. Additionally, many client APIs for developing web maps, such as OpenLayers and Leaflet, support WMS as web map layers. Desktop GIS programs like QGIS support the use of WMS as layers. In fact, the buttons 
allow you to add (from left to right) WMS, WCS, and WFS web services as layers in QGIS.
The GeoServices REST Specification
In 2010, the proprietary GIS software company Esri released an open document called the GeoServices REST Specification (now the GeoServices API) describing the request and response patterns used by default for its ArcGIS Server web services. (Although I mentioned that you can enable OGC communication on an ArcGIS Server service, this option is not the default.)
The GeoServices REST Specification uses a RESTful pattern of communication with the web services. Whereas the OGC services offer a simple list of methods that typically return XML (when they're not returning an image), REST exposes information in a navigable hierarchy of URLs that can return focused packets of HTML, JSON, images, or other data types. For example, using REST and the GeoServices REST specification, you can invoke http://<server>/MapServer/layers/ to get a list of layers in a map service and
Although ArcGIS Server had already been using this pattern of communication for several years, Esri eventually placed the specification online as an open document so that developers would feel encouraged (or at least legally allowed) to implement clients and servers supporting this pattern.
Some controversy around the specification erupted in 2013 when a slightly modified version of the document (with the Esri references scrubbed out) was submitted to a vote for adoption as an OGC specification. Proponents, including Esri, argued that the OGC lacked a nimble, RESTful alternative to the W*S specifications (this lesson previously linked to a 2013 Esri User Conference Q & A item elaborating this argument, but this document has since been taken offline). Opponents expressed concern about functional overlap with existing service types, and claimed that OGC adoption of the specification would give Esri an unfair advantage in competing for contracts. The uproar was significant enough to get the specification pulled from the voting process.
Although the GeoServices REST Specification is not an OGC specification, I am mentioning it in this lesson as an example of a specification that was voluntarily made open by a proprietary software vendor for the purposes of encouraging interoperability and the proliferation of the vendor's platform. It is likely that you will encounter this specification at some point in your career if you do any development with web services that somebody has hosted on ArcGIS Server.