A domain is a website's address. Each of these addresses represents the same location on the internet, but they are different domains because they use different IP addresses to do so. Having multiple domains pointing to one IP address enables companies to host several websites on one physical server without each website appearing to be hosted by the same company (this is called "URL masking").
Domain-specific topics are usually technical in nature and may not be considered part of the software development industry as a whole. Articles for domain developers will typically contain examples and code related to that domain.