Of course, conductors are the only members of the orchestra who do not make a sound. Even so, in modern orchestras conductors are usually the most celebrated figures on the concert platform. Their interpretative role is crucial to our understanding of the music. Any composer presenting a new piece must develop a good relationship and understanding with the conductor.
Conductors as we know them today only emerged in the mid 19th century. Wagner, in particular, developed the role of the conductor as an individual, interpretative figure. Up to that point, conductors usually just kept time and often directed the music from the keyboard.
Here are a couple of instructional websites which give some basic information about conducting techniques: What to think about when conducting | 'Essentials in conducting'