It makes sense that a manager is someone who "manages," or takes charge of something. A baseball team manager is in charge of running the team, while a business manager oversees finances, scheduling, and the everyday operation of the business.
The probable origin of the word manager comes from the Latin manus, meaning "hand." A good manager provides the necessary "hand," guiding others. The Italian maneggiare means "to control," and was especially used with reference to training horses, a job for which certain managers you've worked for might be better suited. The word extends beyond human beings as well: the file manager on your computer is a program designed to organize data.