Human resources is a people business - quite literally. The HR department in any large firm is responsible for all matters concerning staff, handling the process that goes from hiring them to paying them to firing them. Everything has to be approved by, agreed with and signed off on by someone senior within HR. Typically, HR managers also define the scope of new positions within the business which require recruiting for, and put together the necessary recruitment campaign.
It's a big job which is a vital component of a successful business. A good HR manager keeps everything ticking over as it should do. A bad one can bring a company to its knees! If you want to be a good one, you'll need a degree, appropriate certification, and a history of working with payroll and in management positions. A little experience in handling conflicts will probably come in handy when you butt heads with middle management, too.