An individual who is in charge of a certain group of tasks, or a certain subset of a company. A manager often has a staff of people who report to him or her.
As an example, a restaurant will often have a front-of-house manager who helps the patrons, and supervises the hosts; or a specific office project can have a manager, known simply as the project manager. Certain departments within a company designate their managers to be line managers, while others are known as staff managers, depending upon the function of the department.