During the performance, the conductor's primary job is to control the speed of the performance by rhythmic movement. The volume is also controlled by raising or lowering the arms. Most of the conductors' work is behind the scenes, explaining what he wants from the orchestra, just as a movie director states what he wants from the actors. A conductor must be capable of playing in the orchestra in several different positions. During the performance, a conductor is of less importance, since the orchestra actually takes its pace from the first violin, who is quite competent to lead the orchestra!

In addition to setting the speed of the music, the conductor provides other input to the performers. The conductor also can adjust the volume of the entire orchestra, a section, or even an individual. A skilled conductor in front of an attentive orchestra can even clue in individuals when they are playing out of tune. Overall, the conductor's job is to make sure that the orchestra plays together and is well-balanced. Jackie is right that part of a composer's job is indeed to portray the composer's ideas to the piece. And he or she does this mainly by means of tempo or volume (including balancing between instruments). However, a conductor is not necessarily restricted to the composer's ideas and often will take some liberties here and there to fit the piece to the orchestra or even simply because the conductor thinks it would sound better done a bit differently. Of course, this is often done in such small ways that usually only the composer and the orchestra themselves would be able to detect the specific changes. It should be noted too that many music scores are not very detailed so often a conductor doesn't know exactly what a composer would have preferred as far as tempo or volume. This is not necessarily a bad thing, though, as it gives the conductor further freedom to experiement.

The conductor's primary job is to conceive a certain vision of a musical work, based on knowledge, experience, and his or her own artistic sense, and then to lead, command, cajole, instruct, drive, beg, badger, enlighten, tease, frighten, inspire -- whatever it takes, depending on personal style and the circumstances -- an orchestra to make that vision a reality. And we're still in rehearsal. What exactly a conductor does in performance is a question not susceptible of any one clear answer. The public commonly believes that the conductor is shaping the performance before their eyes, and if he stopped, even accidentally, the music would stop, but in fact most orchestras can carry on quite nicely while the conductor falls off the podium and then gets back up -- it's happened. (And of course it used to be common and is still not rare for pianists to conduct from the piano -- Barenboim, e.g. -- which means the orchestra is sometimes getting no conducting at all.) Or it's supposed that she has to beat time, obviously or subtly, to keep the players together, but that's not at all true of a professional group -- the one thing they can do for sure is play in time. Or that individual players need cues to come in at the right time, which is also not true of professionals. So why is it that conductors in fact do all these things, and a lot more that look like attending to details, when the players can attend to the details quite well? It seems to be a combination, in many cases, of reminding the players of things -- subtleties, nuances, shadings already arranged in rehearsal -- and acting out, embodying what's desired, so that the orchestra has before it something like a rhythmic, highly idiosyncratic one-person dance, ranging from formal and dignified, perhaps almost motionless, at one end of the scale, to emotional and gesticulatory to the point of almost leaving the floor at the other. What's fun about this part of the question is that no matter what you decide, it can be contradicted. A conductor signals every aspect of the performance, so the musicians can read what they're to do? Then what are we to make of, for example, Pierre Monteux, who (like a number of conductors) by the end of his career was hardly moving his arms and still bringing audiences to their feet? Well then, the conductor arranges the performance in rehearsal and merely gives reminders in performance. Then what do we make of Bernstein, visibly living and dying with every note, seeming to occupy a space 27 times the size of his actual body, and drawing not only audiences into his volcanic emotional space, but also experienced professional musicians, who can be a pretty cynical lot? And just about every spot on the scale between the two is filled. Really, it occurs to me now, the question of what a conductor does should begin with the question of what an orchestra is, because a conductor is first a musician capable of forming an artistic vision of a piece of music, and then is the person who undertakes to live out a relationship between herself, that music, and the exotic, complicated organism called an orchestra -- and it's that last where all the complications arise. And if you go one step past the obvious when asking that one, you're right into What is music to us? and What changes when we make it in groups? Well Bob, I think our time is about up . . . . Yes, Ray, it surely is, but anyone who's read this far deserves a reward, so how about a couple conductor jokes? No. 1 -- ancient -- category I Could Have Been a Plumber: Orchestra is suffering through long rehearsal under utterly incompetent conductor. Finally a percussionist can stand it no longer. He takes a cymbal and throws it to the floor. Whole place freezes. Conductor draws himself up, looks imperiously around, says, "All right. Who did that?" No. 2 -- not really a joke exactly, less ancient, regret I've forgotten where I read it -- category There's Only So Much You Can Do: [Conductor rehearsing the Mahler First]: No, no, the woodwinds, not like that. It's not wrong, but you're not capturing it. You have to realize what nature meant to Mahler, what it meant to the Germanic soul -- the woods, the earth, springtime, how this affects the musical voice, what birds mean in relation to this key, the timbre, the leaves, the modulation, animal life . . . . [First oboe}: Maestro, do you want it louder? . . . or softer?

Along with what the previous people said which is very true... I can only add the following. There are many nuances that cannot be expressed on printed ink. This is what the musicians read and interpret while they convery the music the audience hears. The conductor MUST know more about the piece he or she is conducting than every member of the orchestra put together. The conductor must, in his interpretation, come closest to what the composer of the music had in mind. When a conductor conducts a symphony or tone poem by a famous non-living composer ( or living actually...except if they're living they can tell the conductor how they want the music played) , he/she must shape the music and convey his feelings through his/her movements and gestures (expression) through the conducting so that the musicians in the orchestra can interpret the conductors feelings and transform them into the music that is heard. Because of this wonderful phenomenon, no two performances of the same piece will every sound the same. This is why people who collect "classical" music have so many recordings of the same piece. Hope this helped :)

I think the following video may help answer the question. Enjoy!

The conductor keeps the orchestra together to sound like "one mind" instead of 100 individual musicians. The conductor keeps the tempo and indicates when to speed up or slow down, give cues to different players or sections when to come in, tells musicians when they are playing too loud or too soft, and gives tips an indicators how to be expressive.