OGC? WMS? WFS? Why must you speak in so many acronyms?
OGC is the Open Geospatial Consortium, the leading standards organization in the Geospatial arena. Somewhat like the W3C, but focused on geospatial applications. The Web Map Server (WMS) specification defines a standard protocol for generating cartographic maps over the web. The Web Feature Server (WFS) defines a standard protocol for querying and retrieving vector features over the web. WMS and WFS are two of the most prominent OGC specifications. GeoServer is the reference implementation of the WFS spec, and also fully implements the WMS spec.
The OGC has defined a complete testing suite for the WMS and WFS specifications as part of their Compliance, Interoperability, Testing and Evaluation CITE project. Prior to the establishment of these tests, implementors of a spec could only guess if they were doing things right. With the CITE testing engine, a developer can run a large suite of tests to make sure that everything is working right. These are very comprehensive tests, and it is not trivial to pass them. GeoServer is the only Open Source server that has successfully passed both the WMS and WFS test suites. We've made every effort to be as easy as possible to set up a standards compliant server, with no additional configuration needed to meet the open standards.
The purpose of all these documents and tests is achieving 'interoperability' – the ability for different pieces of software to successfully interact with one another. We are looking to help build 'The GeoSpatial Web', similar in scope to the World Wide Web (WWW), where someone just needs a Web Server, such as Apache or IIS, and then others can start interacting with their information. So too for geospatial do you need servers and clients that speak the same language, so we're not all just stuck in one way of doing things. This in turn will enable far more innovation, since it can come from all directions, just as the WWW has done, and is what keeps us excited about GeoServer. We hope for GeoServer to serve as a sort of 'Apache for the geospatial web', in that it implements the standards and keeps everyone else honest about adding all kinds of bells and whistles. And it allows anyone to easily get on the web, since it's freely available and easy to install.