Human Resources covers a range of topics, all dealing with the "People" aspects of running a business. HR has 2 major components HRM (management of people) and HRD (learning and development for staff). Typically larger companies have their own HR department but may buy in specific services from a specialist firm. Smaller companies may opt to have all their HR managed by an external HR company.
Human_Resources contains sites for companies specializing in Human Resource services. This may be as an HR service provider to other companies or as a provider of advice/resources/consultancy to HR departments.