Title VII of the civil rights act of 1964 protect employees against sexual harassment in the workplace. What are the two forms of sexual harassment?

Title VII of the civil rights act of 1964 protect employees against sexual harassment in the workplace. What are the two forms of sexual harassment? What if an employer doesn't know about the supervisors harassment of an employee can the employer still be held liable? What defense can an employer make in that situation, and what are the elements of it? What if the sexual-harassment is done by a customer-can the employer ever be liable for that, and if so, under what circumstances?