I have edited your question a bit, but I am not clear on what "their contents" means. There are many functions in today's HR department, so let's talk about that. I do want to caution you, though, that different organizations may vary somewhat in how they use their HR resources.
Often, HR departments are responsible for hiring, discipline, and termination. In some companies, this department makes these determinations on its own, while in other companies, a manager or someone higher in the company will make final determinations, and the HR department will simply screen and/or gather information to assist the decision-maker.
Often, HR departments are responsible for training employees initially or for providing continuing education for employees. Some companies do outsource this, though.
Another function of the HR department is often the administration of unemployment compensation. This might include everything from vetting a termination decision to see if it will result in a UC claim to representing the company at an unemployment compensation hearing.
Still another function of the HR department is the administration of state and federal statutes, for example, the Civil Rights Act, OSHA, and the FLSA.
HR departments are often responsible for the administration of benefits, including sick and vacation leave, and employer-provided life, health, or disability insurance.
That is not a comprehensive list, but I hope enough to get you thinking.