Unlike a hostile work environment, which can involve co-workers and pretty much anyone at work, another type of sexual harassment only comes from above -- it involves someone in a position of power over others at work.
In quid pro quo harassment, a person in some kind of supervisory or management position subjects an employee under his or her authority to unwanted sexual behavior in exchange for some benefit on the job. Essentially, the person in authority is offering to trade his or her ability to improve the harassed employee's position for sexual favors of some kind.
What does this look like in practice?
A job candidate might come into the office to talk with the hiring manager about an open position. The manager alludes to the fact that there are several other qualified candidates already asking about the same position. He or she then suggestively touches the job candidate's shoulder and says, "Exactly how badly do you want this position?" Even though the candidate isn't even working for the manager yet, the situation clearly puts the manager in a position of power.
Another situation might involve an employee who simply wants to change from the night shift to the day shift to make child care easier. When he or she approaches the company's manager, the manager seems receptive -- at first. Then the manager asks the employee for a date. When the employee turns the manager down, the manager's tone changes and the employee is told the shift change isn't possible. The manager says something like, "Maybe you'll be friendlier to me next time you want something."
You could also have a situation where an employee is on a business trip and out to dinner with the boss. After dinner, the boss leans in and tries to kiss the employee. The employee steps back and declines the kiss. The boss responds, "I thought you liked working for me," which is a clear threat to the employee's position.
While all these situations take place in different environments and over different stakes, they're all the same kind of sexual harassment. Your situation may not be as clear as any of these -- it often takes an attorney who handles sexual harassment cases to really determine if you have a valid claim.