Human relationships are like family, friends, or even co-workers or bosses/employees. It's a connection between two people. Human relation an mean the same thing, but its most often used in a professional setting. A human relations, or HR, department at a company is responsible for maintaining a professional atmosphere within a company and often times responsible for maintaining the appearance of professionalism publicly in cases of scandal.
Also, if you have a relationship with someone, it means you have some kind if connection with them, and can be used informally to imply a romantic relationship with the person. If you say you are having relations with someone, it's a little bit outdated, but it has the connotation that you're having sex with that person.